Given below is a list of most popular Database interview questions and answers for your reference.
Q #1) What do you understand by ‘Database’?
Answer: Database is an organized collection of related data where the data is stored and organized to serve some specific purpose.
For Example, A librarian maintain a database of all the information related to the books that are available in the library.
Q #2) Define DBMS.
Answer: DBMS stands for Database Management System. It is a collection of application programs which allow the user to organize, restore and retrieve information about data efficiently and as effectively as possible.
Some of the popular DBMS’s are MySql, Oracle, Sybase, etc.
Q #3) Define RDBMS.
Answer: Relational Database Management System(RDBMS) is based on a relational model of data that is stored in databases in separate tables and they are related to the use of a common column. Data can be accessed easily from the relational database using Structured Query Language (SQL).
Q #4) Enlist the advantages of DBMS.
Answer: The advantages of DBMS includes:
- Data is stored in a structured way and hence redundancy is controlled.
- Validates the data entered and provide restrictions on unauthorized access to the database.
- Provides backup and recovery of the data when required.
- It provides multiple user interfaces.
Q #5) What do you understand by Data Redundancy?
Answer: Duplication of data in the database is known as data redundancy. As a result of data redundancy, duplicated data is present at multiple locations, hence it leads to wastage of the storage space and the integrity of the database is destroyed.
Q #6) What are the various types of relationships in Database? Define them.
Answer: There are 3 types of relationships in Database:
- One-to-one: One table has a relationship with another table having the similar kind of column. Each primary key relates to only one or no record in the related table.
- One-to-many: One table has a relationship with another table that has primary and foreign key relations. The primary key table contains only one record that relates to none, one or many records in the related table.
- Many-to-many: Each record in both the tables can relate to many numbers of records in another table.
Q #7) Explain Normalization and De-Normalization.
Answer:
Normalization is the process of removing redundant data from the database by splitting the table in a well-defined manner in order to maintain data integrity. This process saves much of the storage space.
De-normalization is the process of adding up redundant data on the table in order to speed up the complex queries and thus achieve better performance.
Q #8) What are the different types of Normalization?
Answer: Different types of Normalization are:
- First Normal Form (1NF): A relation is said to be in 1NF only when all the entities of the table contain unique or atomic values.
- Second Normal Form (2NF): A relation is said to be in 2NF only if it is in 1NF and all the non-key attribute of the table is fully dependent on the primary key.
- Third Normal Form (3NF): A relation is said to be in 3NF only if it is in 2NF and every non-key attribute of the table is not transitively dependent on the primary key.
Q #9) What is BCNF?
Answer: BCNF is the Boyce Code Normal form. It is the higher version of 3Nf which does not have any multiple overlapping candidate keys.
Q #10) What is SQL?
Answer: Structured Query language, SQL is an ANSI(American National Standard Institute) standard programming language that is designed specifically for storing and managing the data in the relational database management system (RDBMS) using all kinds of data operations.
Q #11) How many SQL statements are used? Define them.
Answer: SQL statements are basically divided into three categories, DDL, DML, and DCL.
They can be defined as:
- Data Definition Language (DDL) commands are used to define the structure that holds the data. These commands are auto-committed i.e. changes done by the DDL commands on the database are saved permanently.
- Data Manipulation Language (DML) commands are used to manipulate the data of the database. These commands are not auto-committed and can be rolled back.
- Data Control Language (DCL) commands are used to control the visibility of the data in the database like revoke access permission for using data in the database.
Q #12) Enlist some commands of DDL, DML, and DCL.
Answer: Data Definition Language (DDL) commands:
- CREATE to create a new table or database.
- ALTER for alteration.
- TRUNCATE to delete data from the table.
- DROP to drop a table.
- RENAME to rename a table.
Data Manipulation Language (DML) commands:
- INSERT to insert a new row.
- UPDATE to update an existing row.
- DELETE to delete a row.
- MERGE for merging two rows or two tables.
Data Control Language (DCL) commands:
- COMMIT to permanently save.
- ROLLBACK to undo the change.
- SAVEPOINT to save temporarily.
Q #13) Define DML Compiler.
Answer: DML compiler translates DML statements in a query language into a low-level instruction and the generated instruction can be understood by Query Evaluation Engine.
Q #14) What is DDL interpreter?
Answer: DDL Interpreter interprets the DDL statements and records the generated statements in the table containing metadata.
Q #15) Enlist the advantages of SQL.
Answer: Advantages of SQL are:
- Simple SQL queries can be used to retrieve a large amount of data from the database very quickly and efficiently.
- SQL is easy to learn and almost every DBMS supports SQL.
- It is easier to manage the database using SQL as no large amount of coding is required.
Q #16) Explain the terms ‘Record’, ‘Field’ and ‘Table’ in terms of database.
Answer:
Record: Record is a collection of values or fields of a specific entity. For Example, An employee, Salary account, etc.
Field: A field refers to an area within a record that is reserved for specific data. For Example, Employee ID.
Table: Table is the collection of records of specific types. For Example, the Employee table is a collection of records related to all the employees.
Q #17) What do you understand by Data Independence? What are its two types?
Answer: Data Independence refers to the ability to modify the schema definition in one level in such a way that it does not affect the schema definition in the next higher level.
The 2 types of Data Independence are:
- Physical Data Independence: It modifies the schema at the physical level without affecting the schema at the conceptual level.
- Logical Data Independence: It modifies the schema at the conceptual level without affecting or causing changes in the schema at the view level.
Q #18) Define the relationship between ‘View’ and ‘Data Independence’.
Answer: View is a virtual table that does not have its data on its own rather the data is defined from one or more underlying base tables.
Views account for logical data independence as the growth and restructuring of base tables are not reflected in views.
Q #19) What are the advantages and disadvantages of views in the database?
Answer: Advantages of Views:
- As there is no physical location where the data in the view is stored, it generates output without wasting resources.
- Data access is restricted as it does not allow commands like insertion, updation, and deletion.
Disadvantages of Views:
- The view becomes irrelevant if we drop a table related to that view.
- Much memory space is occupied when the view is created for large tables.
Q #20) What do you understand by Functional dependency?
Answer: A relation is said to be in functional dependency when one attribute uniquely defines another attribute.
For Example, R is a Relation, X and Y are two attributes. T1 and T2 are two tuples. Then,
T1[X]=T2[X] and T1[Y]=T2[Y]
Means, the value of component X uniquely define the value of component Y.
Also, X->Y means Y is functionally dependent on X.
Q #21) When is functional dependency said to be the fully functional dependent?
Answer: To fulfill the criteria of fully functional dependency, the relation must meet the requirement of functional dependency.
A functional dependency ‘A’ and ‘B’ are said to be fully functional dependent when removal of any attribute say ‘X’ from ‘A’ means the dependency does not hold anymore.
Q #22) What do you understand by the E-R model?
Answer: E-R model is an Entity-Relationship model which defines the conceptual view of the database.
The E-R model basically shows the real-world entities and their association/relations. Entities here represent the set of attributes in the database.
Q #23) Define Entity, Entity type, and Entity set.
Answer:
Entity can be anything, be it a place, class or object which has an independent existence in the real world.
Entity Type represents a set of entities that have similar attributes.
Entity Set in the database represents a collection of entities having a particular entity type.
Q #24) Define a Weak Entity set.
Answer: Weak Entity set is the one whose primary key comprises its partial key as well as the primary key of its parent entity. This is the case because the entity set may not have sufficient attributes to form a primary key.
Q #25) Explain the terms ‘Attribute’ and ‘Relations’
Answer:
Attribute is described as the properties or characteristics of an entity. For Example, Employee ID, Employee Name, Age, etc., can be attributes of the entity Employee.
Relation is a two-dimensional table containing a number of rows and columns where every row represents a record of the relation. Here, rows are also known as ‘Tuples’ and columns are known as ‘Attributes’.
Q #26) What are VDL and SDL?
Answer: VDL is View Definition Language which represents user views and their mapping to the conceptual schema.
SDL is Storage Definition Language which specifies the mapping between two schemas.
Q #27) Define Cursor and its types.
Answer: Cursor is a temporary work area that stores the data, as well as the result set, occurred after manipulation of data retrieved. A cursor can hold only one row at a time.
The 2 types of Cursor are:
- Implicit cursors are declared automatically when DML statements like INSERT, UPDATE, DELETE is executed.
- Explicit cursors have to be declared when SELECT statements that are returning more than one row are executed.
Q #28) What is the Database transaction?
Answer: Sequence of operation performed which changes the consistent state of the database to another is known as the database transaction. After the completion of the transaction, either the successful completion is reflected in the system or the transaction fails and no change is reflected.
Q #29) Define Database Lock and its types.
Answer: Database lock basically signifies the transaction about the current status of the data item i.e. whether that data is being used by other transactions or not at the present point of time.
There are two types of Database lock: Shared Lock and Exclusive Lock.
Q #30) What is Data Warehousing?
Answer: The storage as well as access to data, that is being derived from the transactions and other sources, from a central location in order to perform the analysis is called Data Warehousing.
Q #31) What do you understand by Join?
Answer: Join is the process of deriving the relationship between different tables by combining columns from one or more tables having common values in each. When a table joins with itself, it is known as Self Join.
Q #32) What do you understand by Index hunting?
Answer: Index hunting is the process of boosting the collection of indexes which helps in improving the query performance as well as the speed of the database.
Q #33) How to improve query performance using Index hunting?
Answer: Index hunting help in improving query performance by:
- Using a query optimizer to coordinate queries with the workload.
- Observing the performance and effect of index and query distribution.
Q #34) Differentiate between ‘Cluster’ and ‘Non-cluster’ index.
Answer: Clustered index alters the table and re-order the way in which the records are stored in the table. Data retrieval is made faster by using the clustered index.
A Non-clustered index does alter the records that are stored in the table but creates a completely different object within the table.
Q #35) What are the disadvantages of a Query?
Answer: Disadvantages of a Query are:
- Indexes are not present.
- Stored procedures are excessively compiled.
- Difficulty in interfacing.
Q #36) What do you understand by Fragmentation?
Answer: Fragmentation is a feature that controls the logical data units, also known as fragments that are stored at different sites of a distributed database system.
Q #37) Define Join types.
Answer: Given below are the types of Join, which are explained with respect to the tables as an Example.
employee table:
employee_info table:
a) Inner JOIN: Inner JOIN is also known as a simple JOIN. This SQL query returns results from both the tables having a common value in rows.
SQL Query:
SELECT * from employee, employee_info WHERE employee.EmpID = employee_info.EmpID ; |
Result:
b) Natural JOIN: This is a type of Inner JOIN that returns results from both the tables having the same data values in the columns of both the tables to be joined.
SQL Query:
SELECT * from employee NATURAL JOIN employee_info; |
Result:
c) Cross JOIN: Cross JOIN returns the result as all the records where each row from the first table is combined with each row of the second table.
SQL Query:
SELECT * from employee CROSS JOIN employee_info; |
Result:
Let us do some modification in the above tables to understand Right JOIN, Left JOIN, and Full JOIN.
employee table:
employee_info table:
a) Right JOIN: Right JOIN is also known as Right Outer JOIN. This returns all the rows as a result from the right table even if the JOIN condition does not match any records in the left table.
SQL Query:
SELECT * from employee RIGHT OUTER JOIN employee_info on (employee.EmpID = employee_info.EmpID); |
Result:
b) Left JOIN: Left JOIN is also known as Left Outer JOIN. This returns all the rows as a result of the left table even if the JOIN condition does not match any records in the right table. This is exactly the opposite of Right JOIN.
SQL Query:
SELECT * from employee LEFT OUTER JOIN employee_info on (employee.EmpID = employee_info.EmpID); |
Result:
c) Outer/Full JOIN: Full JOIN return results in combining the result of both the Left JOIN and Right JOIN.
SQL Query:
SELECT * from employee FULL OUTER JOIN employee_info on (employee.EmpID = employee_info.EmpID); |
Result:
Q #38) What do you understand by ‘Atomicity’ and ‘Aggregation’?
Answer: Atomicity is the condition where either all the actions of the transaction are performed or none. This means, when there is an incomplete transaction, the database management system itself will undo the effects done by the incomplete transaction.
Aggregation is the concept of expressing the relationship with the collection of entities and their relationships.
Q #39) Define Phantom deadlock.
Answer: Phantom deadlock detection is the condition where the deadlock does not actually exist but due to a delay in propagating local information, deadlock detection algorithms identify the deadlocks.
Q #40) Define checkpoint.
Answer: Checkpoint declares a point before which all the logs are stored permanently in the storage disk and is the inconsistent state. In the case of crashes, the amount of work and time is saved as the system can restart from the checkpoint.
Q #41) What is Database partitioning?
Answer: Database partitioning is the process of partitioning tables, indexes into smaller pieces in order to manage and access the data at a finer level.
This process of partitioning reduces the cost of storing a large amount of data as well as enhances the performance and manageability.
Q #42) Explain the importance of Database partitioning.
Answer: The importance of Database partitioning are:
- Improves query performance and manageability.
- Simplifies common administration tasks.
- Acts as a key tool for building systems with extremely high availability requirements.
- Allows accessing a large part of a single partition.
Q #43) Explain the Data Dictionary.
Answer: Data dictionary is a set of information describing the content and structure of the tables and database objects. The job of the information stored in the data dictionary is to control, manipulate and access the relationship between database elements.
Q #44) Explain the Primary Key and Composite Key.
Answer: Primary Key is that column of the table whose every row data is uniquely identified. Every row in the table must have a primary key and no two rows can have the same primary key. Primary key value can never be null nor can it be modified or updated.
Composite Key is a form of the candidate key where a set of columns will uniquely identify every row in the table.
Q #45) What do you understand by the Unique key?
Answer: A Unique key is the same as the primary key whose every row data is uniquely identified with a difference of null value i.e. Unique key allows one value as a NULL value.
Q #46) What do you understand by Database Triggers?
Answer: A set of commands that automatically get executed when an event like Before Insert, After Insert, On Update, On Delete of row occurs in a table is called as Database trigger.
Q #47) Define Stored procedures.
Answer: A Stored procedure is a collection of pre-compiled SQL Queries, which when executed denotes a program taking input, process and gives the output.
Q #48) What do you understand by B-Trees?
Answer: B-Tree represents the data structure in the form of a tree for external memory that reads and writes large blocks of data. It is commonly used in databases and file systems where all the insertions, deletions, sorting, etc., are done in logarithmic time.
Q #49) Name the different data models that are available for database systems.
Answer: Different data models are:
- Relational model
- Network model
- Hierarchical model
Q #50) Differentiate between ‘DELETE’, ‘TRUNCATE’ and ‘DROP’ commands.
Answer: After the execution of ‘DELETE’ operation, COMMIT and ROLLBACK statements can be performed to retrieve the lost data.
After the execution of ‘TRUNCATE’ operation, COMMIT, and ROLLBACK statements cannot be performed to retrieve the lost data.
‘DROP’ command is used to drop the table or key like the primary key/foreign key.
Q #51) Based on the given table, solve the following queries.
Employee table
a) Write the SELECT command to display the details of the employee with empid as 1004.
SQL Query:
SELECT empId, empName, Age, Address from Employee WHERE empId = 1004; |
Result:
b) Write the SELECT command to display all the records of table Employees.
SQL Query:
SELECT * from Employee; |
Result:
c) Write the SELECT command to display all the records of the employee whose name starts with the character ‘R’.
SQL Query:
SELECT * from Employee WHERE empName LIKE ‘R%’; |
Result:
d) Write a SELECT command to display id, age and name of the employees with their age in both ascending and descending order.
SQL Query:
SELECT empId, empName, Age from Employee ORDER BY Age; |
Result:
SELECT empId, empName, Age from Employee ORDER BY Age Desc; |
Result:
e) Write the SELECT command to calculate the total amount of salary on each employee from the below Emp table.
Emp table:
SQL Query:
SELECT empName, SUM(Salary) from Emp GROUP BY empName; |
Result:
Top 50 SQL Interview Questions and Answers (2021 Update)
SQL stands for Structured Query Language is a domain specific programming language for managing the data in Database Management Systems. SQL programming skills are highly desirable and required in the market, as there is a massive use of Database Management Systems (DBMS) in almost every software application. In order to get a job, candidates need to crack the interview in which they are asked various SQL interview questions.
Following is a curated list of SQL interview questions and answers, which are likely to be asked during the SQL interview. Candidates are likely to be asked basic SQL interview questions to advance level SQL questions depending on their experience and various other factors. The below list covers all the SQL interview questions for freshers as well as SQL interview questions for experienced level candidates and some SQL query interview questions.
SQL Interview Questions PDF file: Download here
Frequently Asked SQL Interview Questions and Answers for Freshers and Experienced
1. What is DBMS?
A Database Management System (DBMS) is a program that controls creation, maintenance and use of a database. DBMS can be termed as File Manager that manages data in a database rather than saving it in file systems.
2. What is RDBMS?
RDBMS stands for Relational Database Management System. RDBMS store the data into the collection of tables, which is related by common fields between the columns of the table. It also provides relational operators to manipulate the data stored into the tables.
Example: SQL Server.
3. What is SQL?
SQL stands for Structured Query Language , and it is used to communicate with the Database. This is a standard language used to perform tasks such as retrieval, updation, insertion and deletion of data from a database.
Standard SQL Commands are Select.
4. What is a Database?
Database is nothing but an organized form of data for easy access, storing, retrieval and managing of data. This is also known as structured form of data which can be accessed in many ways.
Example: School Management Database, Bank Management Database.
5. What are tables and Fields?
A table is a set of data that are organized in a model with Columns and Rows. Columns can be categorized as vertical, and Rows are horizontal. A table has specified number of column called fields but can have any number of rows which is called record.
Example:.
Table: Employee.
Field: Emp ID, Emp Name, Date of Birth.
Data: 201456, David, 11/15/1960.
6. What is a primary key?
A primary key is a combination of fields which uniquely specify a row. This is a special kind of unique key, and it has implicit NOT NULL constraint. It means, Primary key values cannot be NULL.
7. What is a unique key?
A Unique key constraint uniquely identified each record in the database. This provides uniqueness for the column or set of columns.
A Primary key constraint has automatic unique constraint defined on it. But not, in the case of Unique Key.
There can be many unique constraint defined per table, but only one Primary key constraint defined per table.
8. What is a foreign key?
A foreign key is one table which can be related to the primary key of another table. Relationship needs to be created between two tables by referencing foreign key with the primary key of another table.
9. What is a join?
This is a keyword used to query data from more tables based on the relationship between the fields of the tables. Keys play a major role when JOINs are used.
10. What are the types of join and explain each?
There are various types of join which can be used to retrieve data and it depends on the relationship between tables.
- Inner Join.
Inner join return rows when there is at least one match of rows between the tables.
- Right Join.
Right join return rows which are common between the tables and all rows of Right hand side table. Simply, it returns all the rows from the right hand side table even though there are no matches in the left hand side table.
- Left Join.
Left join return rows which are common between the tables and all rows of Left hand side table. Simply, it returns all the rows from Left hand side table even though there are no matches in the Right hand side table.
- Full Join.
Full join return rows when there are matching rows in any one of the tables. This means, it returns all the rows from the left hand side table and all the rows from the right hand side table.
11. What is normalization?
Normalization is the process of minimizing redundancy and dependency by organizing fields and table of a database. The main aim of Normalization is to add, delete or modify field that can be made in a single table.
12. What is Denormalization.
DeNormalization is a technique used to access the data from higher to lower normal forms of database. It is also process of introducing redundancy into a table by incorporating data from the related tables.
13. What are all the different normalizations?
The normal forms can be divided into 5 forms, and they are explained below -.
- First Normal Form (1NF):.
This should remove all the duplicate columns from the table. Creation of tables for the related data and identification of unique columns.
- Second Normal Form (2NF):.
Meeting all requirements of the first normal form. Placing the subsets of data in separate tables and Creation of relationships between the tables using primary keys.
- Third Normal Form (3NF):.
This should meet all requirements of 2NF. Removing the columns which are not dependent on primary key constraints.
- Fourth Normal Form (4NF):.
Meeting all the requirements of third normal form and it should not have multi- valued dependencies.
14. What is a View?
A view is a virtual table which consists of a subset of data contained in a table. Views are not virtually present, and it takes less space to store. View can have data of one or more tables combined, and it is depending on the relationship.
15. What is an Index?
An index is performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and it will be faster to retrieve data.
16. What are all the different types of indexes?
There are three types of indexes -.
- Unique Index.
This indexing does not allow the field to have duplicate values if the column is unique indexed. Unique index can be applied automatically when primary key is defined.
- Clustered Index.
This type of index reorders the physical order of the table and search based on the key values. Each table can have only one clustered index.
- NonClustered Index.
NonClustered Index does not alter the physical order of the table and maintains logical order of data. Each table can have 999 nonclustered indexes.
17. What is a Cursor?
A database Cursor is a control which enables traversal over the rows or records in the table. This can be viewed as a pointer to one row in a set of rows. Cursor is very much useful for traversing such as retrieval, addition and removal of database records.
18. What is a relationship and what are they?
Database Relationship is defined as the connection between the tables in a database. There are various data basing relationships, and they are as follows:.
- One to One Relationship.
- One to Many Relationship.
- Many to One Relationship.
- Self-Referencing Relationship.
19. What is a query?
A DB query is a code written in order to get the information back from the database. Query can be designed in such a way that it matched with our expectation of the result set. Simply, a question to the Database.
20. What is subquery?
A subquery is a query within another query. The outer query is called as main query, and inner query is called subquery. SubQuery is always executed first, and the result of subquery is passed on to the main query.
21. What are the types of subquery?
There are two types of subquery – Correlated and Non-Correlated.
A correlated subquery cannot be considered as independent query, but it can refer the column in a table listed in the FROM the list of the main query.
A Non-Correlated sub query can be considered as independent query and the output of subquery are substituted in the main query.
22. What is a stored procedure?
Stored Procedure is a function consists of many SQL statement to access the database system. Several SQL statements are consolidated into a stored procedure and execute them whenever and wherever required.
23. What is a trigger?
A DB trigger is a code or programs that automatically execute with response to some event on a table or view in a database. Mainly, trigger helps to maintain the integrity of the database.
Example: When a new student is added to the student database, new records should be created in the related tables like Exam, Score and Attendance tables.
24. What is the difference between DELETE and TRUNCATE commands?
DELETE command is used to remove rows from the table, and WHERE clause can be used for conditional set of parameters. Commit and Rollback can be performed after delete statement.
TRUNCATE removes all rows from the table. Truncate operation cannot be rolled back.
25. What are local and global variables and their differences?
Local variables are the variables which can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred or used. Variables can be created whenever that function is called.
Global variables are the variables which can be used or exist throughout the program. Same variable declared in global cannot be used in functions. Global variables cannot be created whenever that function is called.
26. What is a constraint?
Constraint can be used to specify the limit on the data type of table. Constraint can be specified while creating or altering the table statement. Sample of constraint are.
- NOT NULL.
- CHECK.
- DEFAULT.
- UNIQUE.
- PRIMARY KEY.
- FOREIGN KEY.
27. What is data Integrity?
Data Integrity defines the accuracy and consistency of data stored in a database. It can also define integrity constraints to enforce business rules on the data when it is entered into the application or database.
28. What is Auto Increment?
Auto increment keyword allows the user to create a unique number to be generated when a new record is inserted into the table. AUTO INCREMENT keyword can be used in Oracle and IDENTITY keyword can be used in SQL SERVER.
Mostly this keyword can be used whenever PRIMARY KEY is used.
29. What is the difference between Cluster and Non-Cluster Index?
Clustered index is used for easy retrieval of data from the database by altering the way that the records are stored. Database sorts out rows by the column which is set to be clustered index.
A nonclustered index does not alter the way it was stored but creates a complete separate object within the table. It point back to the original table rows after searching.
30. What is Datawarehouse?
Datawarehouse is a central repository of data from multiple sources of information. Those data are consolidated, transformed and made available for the mining and online processing. Warehouse data have a subset of data called Data Marts.
31. What is Self-Join?
Self-join is set to be query used to compare to itself. This is used to compare values in a column with other values in the same column in the same table. ALIAS ES can be used for the same table comparison.
32. What is Cross-Join?
Cross join defines as Cartesian product where number of rows in the first table multiplied by number of rows in the second table. If suppose, WHERE clause is used in cross join then the query will work like an INNER JOIN.
33. What is user defined functions?
User defined functions are the functions written to use that logic whenever required. It is not necessary to write the same logic several times. Instead, function can be called or executed whenever needed.
34. What are all types of user defined functions?
Three types of user defined functions are.
- Scalar Functions.
- Inline Table valued functions.
- Multi statement valued functions.
Scalar returns unit, variant defined the return clause. Other two types return table as a return.
35. What is collation?
Collation is defined as set of rules that determine how character data can be sorted and compared. This can be used to compare A and, other language characters and also depends on the width of the characters.
ASCII value can be used to compare these character data.
36. What are all different types of collation sensitivity?
Following are different types of collation sensitivity -.
- Case Sensitivity – A and a and B and b.
- Accent Sensitivity.
- Kana Sensitivity – Japanese Kana characters.
- Width Sensitivity – Single byte character and double byte character.
37. Advantages and Disadvantages of Stored Procedure?
Stored procedure can be used as a modular programming – means create once, store and call for several times whenever required. This supports faster execution instead of executing multiple queries. This reduces network traffic and provides better security to the data.
Disadvantage is that it can be executed only in the Database and utilizes more memory in the database server.
38. What is Online Transaction Processing (OLTP)?
Online Transaction Processing (OLTP) manages transaction based applications which can be used for data entry, data retrieval and data processing. OLTP makes data management simple and efficient. Unlike OLAP systems goal of OLTP systems is serving real-time transactions.
Example – Bank Transactions on a daily basis.
39. What is CLAUSE?
SQL clause is defined to limit the result set by providing condition to the query. This usually filters some rows from the whole set of records.
Example – Query that has WHERE condition
Query that has HAVING condition.
40. What is recursive stored procedure?
A stored procedure which calls by itself until it reaches some boundary condition. This recursive function or procedure helps programmers to use the same set of code any number of times.
41. What is Union, minus and Interact commands?
UNION operator is used to combine the results of two tables, and it eliminates duplicate rows from the tables.
MINUS operator is used to return rows from the first query but not from the second query. Matching records of first and second query and other rows from the first query will be displayed as a result set.
INTERSECT operator is used to return rows returned by both the queries.
42. What is an ALIAS command?
ALIAS name can be given to a table or column. This alias name can be referred in WHERE clause to identify the table or column.
Example-.
Select st.StudentID, Ex.Result from student st, Exam as Ex where st.studentID = Ex. StudentID
Here, st refers to alias name for student table and Ex refers to alias name for exam table.
43. What is the difference between TRUNCATE and DROP statements?
TRUNCATE removes all the rows from the table, and it cannot be rolled back. DROP command removes a table from the database and operation cannot be rolled back.
44. What are aggregate and scalar functions?
Aggregate functions are used to evaluate mathematical calculation and return single values. This can be calculated from the columns in a table. Scalar functions return a single value based on the input value.
Example -.
Aggregate – max(), count - Calculated with respect to numeric.
Scalar – UCASE(), NOW() – Calculated with respect to strings.
45. How can you create an empty table from an existing table?
Example will be -.
Select * into studentcopy from student where 1=2
Here, we are copying student table to another table with the same structure with no rows copied.
46. How to fetch common records from two tables?
Common records result set can be achieved by -.
Select studentID from student INTERSECT Select StudentID from Exam
47. How to fetch alternate records from a table?
Records can be fetched for both Odd and Even row numbers -.
To display even numbers-.
Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=0
To display odd numbers-.
Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=1
from (Select rowno, studentId from student) where mod(rowno,2)=1.[/sql]
48. How to select unique records from a table?
Select unique records from a table by using DISTINCT keyword.
Select DISTINCT StudentID, StudentName from Student.
49. What is the command used to fetch first 5 characters of the string?
There are many ways to fetch first 5 characters of the string -.
Select SUBSTRING(StudentName,1,5) as studentname from student
Select LEFT(Studentname,5) as studentname from student
50. Which operator is used in query for pattern matching?
LIKE operator is used for pattern matching, and it can be used as -.
- % - Matches zero or more characters.
- _(Underscore) – Matching exactly one character.
1) Define Database.
A prearranged collection of figures known as data is called database.
2) What is DBMS?
Database Management Systems (DBMS) are applications designed especially which enable user interaction with other applications.
3) What are the various kinds of interactions catered by DBMS?
The various kind of interactions catered by DBMS are:
- Data definition
- Update
- Retrieval
- Administration
4) Segregate database technology's development.
The development of database technology is divided into:
- Structure or data model
- Navigational model
- SQL/ relational model
5) Who proposed the relational model?
Edgar F. Codd proposed the relational model in 1970.
6) What are the features of Database language?
A database language may also incorporate features like:
DBMS-specific Configuration and management of storage engine
Computations to modification of query results by computations, like summing, counting, averaging, grouping, sorting and cross-referencing Constraint enforcement Application Programming Interface
7) What do database languages do?
As special-purpose languages, they have:
- Data definition language
- Data manipulation language
- Query language
8) Define database model.
A data model determining fundamentally how data can be stored, manipulated and organised and the structure of the database logically is called database model.
9) What is SQL?
Structured Query Language (SQL) being ANSI standard language updates database and commands for accessing.
10) Enlist the various relationships of database.
The various relationships of database are:
- One-to-one: Single table having drawn relationship with another table having similar kind of columns.
- One-to-many: Two tables having primary and foreign key relation.
- Many-to-many: Junction table having many tables related to many tables.
11) Define Normalization.
Organized data void of inconsistent dependency and redundancy within a database is called normalization.
12) Enlist the advantages of normalizing database.
Advantages of normalizing database are:
- No duplicate entries
- Saves storage space
- Boasts the query performances.
13) Define Denormalization.
Boosting up database performance, adding of redundant data which in turn helps rid of complex data is called denormalization.
14) Define DDL and DML.
Managing properties and attributes of database is called Data Definition Language(DDL).
Manipulating data in a database such as inserting, updating, deleting is defined as Data Manipulation Language. (DML)
15) Enlist some commands of DDL.
They are:
CREATE:
Create is used in the CREATE TABLE statement. Syntax is:
CREATE TABLE [column name] ( [column definitions] ) [ table parameters]
ALTER:
It helps in modification of an existing object of database. Its syntax is:
ALTER objecttype objectname parameters.
DROP:
It destroys an existing database, index, table or view. Its syntax is:
DROP objecttype objectname.
16) Define Union All operator and Union.
Full recordings of two tables is Union All operator.A distinct recording of two tables is Union.
17) Define cursor.
A database object which helps in manipulating data row by row representing a result set is called cursor.
18) Enlist the cursor types.
They are:
- Dynamic: it reflects changes while scrolling.
- Static: doesn't reflect changes while scrolling and works on recording of snapshot.
- Keyset: data modification without reflection of new data is seen.
19) Enlist the types of cursor.
They types of cursor are:
- Implicit cursor: Declared automatically as soon as the execution of SQL takes place without the awareness of the user.
- Explicit cursor: Defined by PL/ SQL which handles query in more than one row.
20) Define sub-query.
A query contained by a query is called Sub-query.
21) Why is group-clause used?
Group-clause uses aggregate values to be derived by collecting similar data.
22) Compare Non-clustered and clustered index
Both having B-tree structure, non-clustered index has data pointers enabling one table many non-clustered indexes while clustered index is distinct for every table.
23) Define Aggregate functions.
Functions which operate against a collection of values and returning single value is called aggregate functions
24) Define Scalar functions.
Scalar function is depended on the argument given and returns sole value.
25) What restrictions can you apply when you are creating views?
Restrictions that are applied are:
- Only the current database can have views.
- You are not liable to change any computed value in any particular view.
- Integrity constants decide the functionality of INSERT and DELETE.
- Full-text index definitions cannot be applied.
- Temporary views cannot be created.
- Temporary tables cannot contain views.
- No association with DEFAULT definitions.
- Triggers such as INSTEAD OF is associated with views.
26) Define "correlated subqueries".
A 'correlated subquery' is a sort of sub query but correlated subquery is reliant on another query for a value that is returned. In case of execution, the sub query is executed first and then the correlated query.
27) Define Data Warehousing.
Storage and access of data from the central location in order to take some strategic decision is called Data Warehousing. Enterprise management is used for managing the information whose framework is known as Data Warehousing.
28) Define Join and enlist its types.
Joins help in explaining the relation between different tables. They also enable you to select data with relation to data in another table.
The various types are:
- INNER JOINs: Blank rows are left in the middle while more than equal to two tables are joined.
- OUTER JOINs: Divided into Left Outer Join and Right Outer Join. Blank rows are left at the specified side by joining tables in other side.
Other joins are CROSS JOINs, NATURAL JOINs, EQUI JOIN and NON-EQUI JOIN.
29) What do you mean by Index hunting?
Indexes help in improving the speed as well as the query performance of database. The procedure of boosting the collection of indexes is named as Index hunting.
30) How does Index hunting help in improving query performance?
Index hunting helps in improving the speed as well as the query performance of database. The followed measures are achieved to do that:
- The query optimizer is used to coordinate the study of queries with the workload and the best use of queries suggested based on this.
- Index, query distribution along with their performance is observed to check the effect.
- Tuning databases to a small collection of problem queries is also recommended.
31) Enlist the disadvantages of query.
The disadvantages of query are:
- No indexes
- Stored procedures are excessively compiled.
- Triggers and procedures are without SET NOCOUNT ON.
- Complicated joins making up inadequately written query.
- Cursors and temporary tables showcase a bad presentation.
32) Enlist ways to efficiently code transactions.
Ways to efficiently code transactions:
- User input should not be allowed while transactions.
- While browsing, transactions must not be opened of data.
- Transactions must be kept as small as possible.
- Lower transaction segregation levels.
- Least information of data must be accessed while transacting.
33) What is Executive Plan?
Executive plan can be defined as:
- SQL Server caches collected procedure or the plan of query execution and used thereafter by subsequent calls.
- An important feature in relation to performance enhancement.
- Data execution plan can be viewed textually or graphically.
34) Define B-trees.
A data structure in the form of tree which stores sorted data and searches, insertions, sequential access and deletions are allowed in logarithmic time.
35) Differentiate Table Scan from Index Scan.
Iterating over all the table rows is called Table Scan while iterating over all the index items is defined as Index Scan.
36) What do you mean by Fill Factor concept with respect to indexes?
Fill Factor can be defined as being that value which defines the percentage of left space on every leaf-level page that is to be packed with data. 100 is the default value of Fill Factor.
37) Define Fragmentation.
Fragmentation can be defined as a database feature of server that promotes control on data which is stored at table level by the user.
38) Differentiate Nested Loop, Hash Join and Merge Join.
Nested loop (loop over loop)
An outer loop within an inner loop is formed consisting of fewer entries and then for individual entry, inner loop is individually processed.
E.g.
- Select col1.*, col2.* from coll, col2 where coll.col1=col2.col2;
It's processing takes place in this way:
For i in (select * from col1) loop
For j in (select * from col2 where col2=i.col1) loop
Results are displayed;
End of the loop;
End of the loop;
The Steps of nested loop are:
- Identify outer (driving) table
- Assign inner (driven) table to outer table.
- For every row of outer table, access the rows of inner table.
Nested Loops is executed from the inner to the outer as:
- outer_loop
- inner_loop
- Hash join
While joining large tables, the use of Hash Join is preferred.
Algorithm of Hash Join is divided into:
- Build: It is a hash table having in-memory which is present on the smaller table.
- Probe: this hash value of the hash table is applicable for each second row element.
- Sort merge join
Two independent sources of data are joined in sort merge join. They performance is better as compared to nested loop when the data volume is big enough but it is not good as hash joins generally.The full operation can be divided into parts of two:
Sort join operation :
Get first row R1 from input1
Get first row R2 from input2.
Merge join operation:
'while' is not present at either loop's end.
if R1 joins with R2
next row is got R2 from the input 2
return (R1, R2)
else if R1 < style=""> next row is got from R1 from input 1
else
next row is got from R2 from input 2
end of the loop
39) What is Database partitioning?
Division of logical database into independent complete units for improving its management, availability and performance is called Database partitioning.
40) Explain the importance of partitioning.
Splitting of one table which is large into smaller database entities logically is called database partitioning. Its benefits are:
- To improve query performance in situations dramatically when mostly rows which are heavily accessed are in one partition.
- Accessing large parts of a single partition
- Slower and cheaper storage media can be used for data which is seldom used.
41) Define Database system.
DBMS along with database is called Database system.
42) What do you mean by Query Evaluation Engine?
Query Evaluation Engine executes the low-level instructions that are generated by the compiler.
43) Define DDL Interpreter.
DDL statements are interpreted and recorded in tables called metadata.
44) Define Atomicity and Aggregation.
Atomicity: It's an all or none concept which enables the user to be assured of incomplete transactions to be taken care of. The actions involving incomplete transactions are left undone in DBMS.
Aggregation: The collected entities and their relationship are aggregated in this model. It is mainly used in expressing relationships within relationships.
45) Enlist the various transaction phases.
The various transaction phases are:
- Analysis Phase.
- Redo Phase
- Undo Phase
46) Define Object-oriented model.
Compilations of objects make up this model in which values are stored within instance variables which is inside the object. The object itself comprises bodies of object for its operation which are called methods. Objects containing same kind of variables and methods are called classes.
47) Define Entity.
It can be defined as being a 'thing' with an independent existence in the real world.
48) What do you mean by Entity type?
A set of entries having similar attributes are entity types.
49) Define Entity Set.
Compilation of all entries of any particular type of entry in the database is called Entity Set.
50) What do you mean by Entity type extension?
Compilation of similar entity types into one particular type which is grouped together as an entity set.
