SQL interview questions

General SQL Interview Questions:

1. What is a Database?

A database is a collection of information in an organized form for faster and better access, storage and manipulation. It can also be defined as a collection of tables, schema, views and other database objects.

2. What is Database Testing?

It is AKA back-end testing or data testing.

Database testing involves in verifying the integrity of data in the front end with the data present in the back end. It validates the schema, database tables, columns, indexes, stored procedures, triggers, data duplication, orphan records, junk records. It involves in updating records in a database and verifying the same on the front end.

3. What is the difference between GUI Testing and Database Testing?

• GUI Testing is AKA User Interface Testing or Front-end testing
  Database Testing is AKA back-end testing or data testing.
• GUI Testing deals with all the testable items that are open to the user to interaction such as Menus, Forms etc.
  Database Testing deals with all the testable items that are generally hidden from the user.
• The tester who is performing GUI Testing doesn’t need to know Structured Query Language
  The tester who is performing Database Testing needs to know Structured Query Language
• GUI Testing includes invalidating the text boxes, check boxes, buttons, drop-downs, forms etc., majorly the look and feel of the overall application
  Database Testing involves in verifying the integrity of data in the front end with the data present in the back end. It validates the schema, database tables, columns, indexes, stored procedures, triggers, data duplication, orphan records, junk records. It involves in updating records in a database and verifying the same on the front end.

4. What is a Table in a Database?

A table is a database object used to store records in a field in the form of columns and rows that holds data.

5. What is a Field in a Database?

A field in a Database table is a space allocated to store a particular record within a table.

6. What is a Record in a Database?

A record (also called a row of data) is an ordered set of related data in a table.

7. What is a column in a Table?

A column is a vertical entity in a table that contains all information associated with a specific field in a table.

8. What is DBMS?

Database Management System is a collection of programs that enables a user to store, retrieve, update and delete information from a database.

9. What is RDBMS?

RDBMS stands for Relational Database Management System. RDBMS is a database management system (DBMS) that is based on the relational model. Data from relational database can be accessed using Structured Query Language (SQL)

10. What are the popular Database Management Systems in the IT Industry?

Oracle, MySQL, Microsoft SQL Server, PostgreSQL, Sybase, MongoDB, DB2, and Microsoft Access etc.,

11. What is SQL?

SQL Overview: SQL stands for Structured Query Language. It is an American National Standard Institute (ANSI) standard. It is a standard language for accessing and manipulating databases. Using SQL, some of the action we could do are to create databases, tables, stored procedures (SP’s), execute queries, retrieve, insert, update, delete data against a database.

12. What are the different types of SQL commands?

SQL commands are segregated into following types:

• DDL – Data Definition Language
• DML – Data Manipulation Language
• DQL – Data Query Language
• DCL – Data Control Language
• TCL – Transaction Control Language

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13. What are the different DDL commands in SQL?

DDL commands are used to define or alter the structure of the database.

• CREATE: To create databases and database objects
• ALTER: To alter existing database objects
• DROP: To drop databases and databases objects
• TRUNCATE: To remove all records from a table but not its database structure
• RENAME: To rename database objects

14. What are the different DML commands in SQL?

DML commands are used for managing data present in the database.

• SELECT: To select specific data from a database
• INSERT: To insert new records into a table
• UPDATE: To update existing records
• DELETE: To delete existing records from a table

15. What are the different DCL commands in SQL?

DCL commands are used to create roles, grant permission and control access to the database objects.

• GRANT: To provide user access
• DENY: To deny permissions to users
• REVOKE: To remove user access

16. What are the different TCL commands in SQL?

TCL commands are used to manage the changes made by DML statements.

• COMMIT: To write and store the changes to the database
• ROLLBACK: To restore the database since the last commit

17. What is an Index?

An index is used to speed up the performance of queries. It makes faster retrieval of data from the table. The index can be created on one column or a group of columns.

18. What is a View?

A view is like a subset of a table which is stored logically in a database. A view is a virtual table. It contains rows and columns similar to a real table. The fields in the view are fields from one or more real tables. Views do not contain data of their own. They are used to restrict access to the database or to hide data complexity.

1	CREATE VIEW view_name AS SELECT column_name1, column_name2 FROM table_name WHERE CONDITION;

19. What are the advantages of Views?

Some of the advantages of Views are

1. Views occupy no space
2. Views are used to simply retrieve the results of complicated queries that need to be executed often.
3. Views are used to restrict access to the database or to hide data complexity.

20. What is a Subquery ? 

A Subquery is a SQL query within another query. It is a subset of a Select statement whose return values are used in filtering the conditions of the main query.

21. What is a temp table?

Ans. A temp table is a temporary storage structure to store the data temporarily.

22. How to avoid duplicate records in a query? 

The SQL SELECT DISTINCT query is used to return only unique values. It eliminates all the duplicated values.

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23. What is the difference between Rename and Alias?

‘Rename’ is a permanent name given to a table or column

‘Alias’ is a temporary name given to a table or column.

24. What is a Join?

Join is a query, which retrieves related columns or rows from multiple tables.

25. What are the different types of joins?

Types of Joins are as follows:

• INNER JOIN
• LEFT JOIN
• RIGHT JOIN
• OUTER JOIN

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26. What is the difference between an inner and outer join?

An inner join returns rows when there is at least some matching data between two (or more) tables that are being compared.

An outer join returns rows from both tables that include the records that are unmatched from one or both the tables.

27. What are SQL constraints?

SQL constraints are the set of rules that enforced some restriction while inserting, deleting or updating of data in the databases.

28. What are the constraints available in SQL?

Some of the constraints in SQL are – Primary Key, Foreign Key, Unique Key, SQL Not Null, Default, Check and Index constraint.

29. What is a Unique constraint?

A unique constraint is used to ensure that there are no duplication values in the field/column.

30. What is a Primary Key?

A PRIMARY KEY constraint uniquely identifies each record in a database table. All columns participating in a primary key constraint must not contain NULL values.

31. Can a table contain multiple PRIMARY KEY’s?

The short answer is no, a table is not allowed to contain multiple primary keys but it allows to have one composite primary key consisting of two or more columns.

32. What is a Composite PRIMARY KEY?

Composite PRIMARY KEY is a primary key created on more than one column (combination of multiple fields) in a table.

33. What is a FOREIGN KEY?

A FOREIGN KEY is a key used to link two tables together. A FOREIGN KEY in a table is linked with the PRIMARY KEY of another table.

34. Can a table contain multiple FOREIGN KEY’s?

A table can have many FOREIGN KEY’s.

35. What is the difference between UNIQUE and PRIMARY KEY constraints?

There should be only one PRIMARY KEY in a table whereas there can be any number of UNIQUE Keys.

PRIMARY KEY doesn’t allow NULL values whereas Unique key allows NULL values.

36. What is a NULL value?

A field with a NULL value is a field with no value. A NULLvalue is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation. Assume, there is a field in a table is optional and it is possible to insert a record without adding a value to the optional field then the field will be saved with a NULL value.

37. What is the difference between NULL value, Zero, and Blank space?

As I mentioned earlier, Null value is field with no value which is different from zero value and blank space.

Null value is a field with no value.

Zero is a number

Blank space is the value we provide. The ASCII value of space is CHAR(32).

38. How to Test for NULL Values?

A field with a NULL value is a field with no value. NULLvalue cannot be compared with other NULL values. Hence, It is not possible to test for NULL values with comparison operators, such as =, <, or <>. For this, we have to use the IS NULL and IS NOT NULL operators.

1	SELECT column_names FROM table_name WHERE column_name IS NULL;

1	SELECT column_names FROM table_name WHERE column_name IS NOT NULL;

39. What is SQL NOT NULL constraint?

NOT NULL constraint is used to ensure that the value in the filed cannot be a NULL

40. What is a CHECK constraint?

A CHECK constraint is used to limit the value that is accepted by one or more columns.

E.g. ‘Age’ field should contain only the value greater than 18.

1	CREATE TABLE EMP_DETAILS(EmpID int NOT NULL, NAME VARCHAR (30) NOT NULL, Age INT CHECK (AGE > 18), PRIMARY KEY (EmpID));

41. What is a DEFAULT constraint?

DEFAULT constraint is used to include a default value in a column when no value is supplied at the time of inserting a record.

42. What is Normalization?

Normalization is the process of table design to minimize the data redundancy. There are different types of Noramalization forms in SQL.

• First Normal Form
• Second Normal Form
• Third Normal Form
• Boyce and Codd Normal Form

43. What is Stored procedure?

A Stored Procedure is a collection of SQL statements that have been created and stored in the database to perform a particular task. The stored procedure accepts input parameters and processes them and returns a single value such as a number or text value or a result set (set of rows).

44. What is a Trigger?

A Trigger is a SQL procedure that initiates an action in response to an event (Insert, Delete or Update) occurs. When a new Employee is added to an Employee_Details table, new records will be created in the relevant tables such as Employee_Payroll, Employee_Time_Sheet etc.,

45. Explain SQL Data Types?

In SQL Server, each column in a database table has a name and a data type. We need to decide what type of data to store inside each and every column of a table while creating a SQL table.

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46. What are the possible values that can be stored in a BOOLEAN data field?

TRUE and FALSE

47. What is the largest value that can be stored in a BYTE data field?

The largest number that can be represented in a single byte is 11111111 or 255. The number of possible values is 256 (i.e. 255 (the largest possible value) plus 1 (zero), or 28).

48. What are Operators available in SQL?

SQL Operator is a reserved word used primarily in an SQL statement’s WHERE clause to perform operations, such as arithmetic operations and comparisons. These are used to specify conditions in an SQL statement.

There are three types of Operators.

1. Arithmetic Operators
2. Comparison Operators
3. Logical Operators

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49. Which TCP/IP port does SQL Server run?

By default it is 1433

50. List out the ACID properties and explain? 

Following are the four properties of ACID. These guarantees that the database transactions are processed reliably.

• Atomicity
• Consistency
• Isolation
• Durability

51. Define the SELECT INTO statement.

The SELECT INTO statement copies data from one table into a new table. The new table will be created with the column-names and types as defined in the old table. You can create new column names using the AS clause.

1	SELECT * INTO newtable FROM oldtable WHERE condition;

52. What is the difference between Delete, Truncate and Drop command?

The difference between the Delete, Truncate and Drop command is

• Delete command is a DML command, it is used to delete rows from a table. It can be rolled back.
• Truncate is a DDL command, it is used to delete all the rows from the table and free the space containing the table. It cant be rolled back.
• Drop is a DDL command, it removes the complete data along with the table structure(unlike truncate command that removes only the rows). All the tables’ rows, indexes, and privileges will also be removed.

53. What is the difference between Delete and Truncate?

The difference between the Delete, and Truncate are

DELETE	TRUNCATE
Delete statement is used to delete rows from a table. It can be rolled back.	Truncate statement is used to delete all the rows from the table and free the space containing the table. It cant be rolled back.
We can use WHERE condition in DELETE statement and can delete required rows	We cant use WHERE condition in TRUNCATE statement. So we cant delete required rows alone
We can delete specific rows using DELETE	We can only delete all the rows at a time using TRUNCATE
Delete is a DML command	Truncate is a DDL command
Delete maintains log and performance is slower than Truncate	Truncate maintains minimal log and performance wise faster
We need DELETE permission on Table to use DELETE command	We need at least ALTER permission on the table to use TRUNCATE command

54. What is the difference between Union and Union All command?

This is one of the tricky SQL Interview Questions. Interviewer may ask you this question in another way as what are the advantages of Union All over Union.

Both Union and Union All concatenate the result of two tables but the way these two queries handle duplicates are different.

Union: It omits duplicate records and returns only distinct result set of two or more select statements.

Union All: It returns all the rows including duplicates in the result set of different select statements.

Performance wise Union All is faster than Union, Since Union All doesn’t remove duplicates. Union query checks the duplicate values which consumes some time to remove the duplicate records.

Assume: Table1 has 10 records, Table2 has 10 records. Last record from both the tables are same.

If you run Union query.

1 2 3

SELECT * FROM Table1 UNION SELECT * FROM Table2

Output: Total 19 records

If you run Union query.

1 2 3

SELECT * FROM Table1 UNION ALL SELECT * FROM Table2

Output: Total 20 records

Data type of all the columns in the two tables should be same.

55. What is the difference between Having and Where clause?

Where clause is used to fetch data from a database that specifies particular criteria whereas a Having clause is used along with ‘GROUP BY’ to fetch data that meets particular criteria specified by the Aggregate functions. Where clause cannot be used with Aggregate functions, but the Having clause can.

56. What are aggregate functions in SQL?

SQL aggregate functions return a single value, calculated from values in a column. Some of the aggregate functions in SQL are as follows

• AVG() – This function returns the average value
• COUNT() – This function returns the number of rows
• MAX() – This function returns the largest value
• MIN() – This function returns the smallest value
• ROUND() – This function rounds a numeric field to the number of decimals specified
• SUM() – This function returns the sum

57. What are string functions in SQL?

SQL string functions are used primarily for string manipulation. Some of the widely used SQL string functions are

• LEN() – It returns the length of the value in a text field
• LOWER() – It converts character data to lower case
• UPPER() – It converts character data to upper case
• SUBSTRING() – It extracts characters from a text field
• LTRIM() – It is to remove all whitespace from the beginning of the string
• RTRIM() – It is to remove all whitespace at the end of the string
• CONCAT() – Concatenate function combines multiple character strings together
• REPLACE() –  To update the content of a string.

Practical SQL Interview Questions:

58. How to add new Employee details in an Employee_Details table with the following details

Employee_Name: John, Salary: 5500, Age: 29?

1	INSERT into Employee_Details (Employee_Name, Salary, Age) VALUES (‘John’, 5500 , 29);

59. How to add a column ‘Salary’ to a table Employee_Details?

1	ALTER TABLE Employee_Details ADD (Salary);

60. How to change a value of the field ‘Salary’ as 7500 for an Employee_Name ‘John’ in a table Employee_Details?

1	UPDATE Employee_Details set Salary = 7500 where Employee_Name = ‘John’;

61. Write an SQL Query to select all records from the table?

1	Select * from table_name;

62. How To Get List of All Tables From A DataBase?

To view the tables available on a particular DataBase

1 2 3 4

USE TestDB GO SELECT * FROM sys.Tables GO

63. Define SQL Delete statement.

The SQL Delete statement is used to delete records from a table.

1	DELETE FROM table_name WHERE some_column=some_value;

64. Write the command to remove all Players named Sachin from the Players table.

1	DELETE from Players WHERE Player_Name = ‘Sachin’