SQL Interview Questions & Answers

Home
SQL Interview Questions & Answers

SQL Interview Questions & Answers

SQL is a standard language for storing, manipulating and retrieving data in databases.

What is SQL ?

SQL Stands for Structured query language.
SQL let you access and manipulate database.
SQL became a standard of the American National Standards Institute (ANSI) in 1986, and of the International Organization for Standardization (ISO) in 1987

What Can SQL do?

SQL can execute queries against a database.
SQL can retrieve data from a database
SQL can insert, update, delete records in a database
SQL can create new databases
SQL can create new tables in a database
SQL can create stored procedures in a database
SQL can create views in a database
SQL can set permissions on tables, procedures, and views

RDBMS

RDBMS stands for Relational Database Management System.

RDBMS is the basis for SQL, and for all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables. A table is a collection of related data entries and it consists of columns and rows.

Look at the "EMPLOYEES" table:

        
            SELECT * FROM EMPLOYEES

SQL Syntax

SQL Statements

Most of the actions you need to perform on a database are done with SQL statements.

Database Tables

A database most often contains one or more tables. Each table is identified by a name (e.g. "Employees" or "Salary"), and contain records (rows) with data.

Note :

SQL keywords are NOT case sensitive: select is the same as SELECT

Semicolon after SQL Statements?

Some database systems require a semicolon at the end of each SQL statement.

Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.

Some of The Most Important SQL Commands

SELECT - extracts data from a database
UPDATE - updates data in a database
DELETE - deletes data from a database
INSERT INTO - inserts new data into a database
CREATE DATABASE - creates a new database
ALTER DATABASE - modifies a database
CREATE TABLE - creates a new table
ALTER TABLE - modifies a table
DROP TABLE - deletes a table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index

SQL SELECT Statement

The SELECT statement is used to select data from a database.

Syntax;

    
        SELECT column1, column2, ...

        FROM table_name;

The SQL SELECT DISTINCT Statement

The SELECT DISTINCT statement is used to return only distinct (different) values.

Syntax:

    
        SELECT DISTINCT column1, column2, ...

        FROM table_name;

Count Distinct

By using the DISTINCT keyword in a function called COUNT, we can return the number of different countries.

Syntax:

    
        SELECT Count(*) AS DistinctCountries

        FROM (SELECT DISTINCT Country FROM Customers);

SQL WHERE Clause

The WHERE clause is used to filter records.

It is used to extract only those records that fulfill a specified condition.

Syntax:

    
        SELECT column1, column2, ...

        FROM table_name

        WHERE condition;

Example:

    
        SELECT emp_name, emp_dob, emp_address

        FROM employee

        WHERE emp_city = 'Mumbai';

Text Fields vs. Numeric Fields

SQL requires single quotes around text values (most database systems will also allow double quotes).

However, numeric fields should not be enclosed in quotes:


    SELECT emp_name, emp_dob, emp_address

    FROM employee

    WHERE emp_code = 10001;

SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set in ascending or descending order.

Syntax:

    
        SELECT column1, column2, ...

        FROM table_name

        ORDER BY column1, column2, ... ASC|DESC;

DESC

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, use the DESC keyword.

Order Alphabetically

For string values the ORDER BY keyword will order alphabetically:

Alphabetically DESC

To sort the table reverse alphabetically, use the DESC keyword:

ORDER BY Several Columns

The following SQL statement selects all employees from the "employees" table, sorted by the "Country" and the "emp_name" column. This means that it orders by Country, but if some rows have the same Country, it orders them by emp_name:

Using Both ASC and DESC

The following SQL statement selects all employees from the "employees" table, sorted ascending by the "Country" and descending by the "emp_name" column:

SQL AND Operator

The SQL AND Operator

The WHERE clause can contain one or many AND operators.

The AND operator is used to filter records based on more than one condition, like if you want to return all employees from Spain that starts with the letter 'G':

Example:
Select all employees from Spain that starts with the letter 'G':

    
        SELECT *

        FROM employees

        WHERE Country = 'Spain' AND emp_name LIKE 'G%';

AND vs OR

The AND operator displays a record if all the conditions are TRUE.

The OR operator displays a record if any of the conditions are TRUE.

All Conditions Must Be True

The following SQL statement selects all fields from employees where Country is "Germany" AND City is "Berlin" AND PostalCode is higher than 12000:

Combining AND and OR

You can combine the AND and OR operators.

The following SQL statement selects all employees from India that starts with a "G" or an "R".

Make sure you use parenthesis to get the correct result.

Example

Select all Indian employees that starts with either "G" or "R":

    
        SELECT * FROM employees

        WHERE Country = 'India' AND (employees LIKE 'G%' OR employees LIKE 'R%');

Without parenthesis, the select statement will return all employees from India that starts with a "G", plus all employees that starts with an "R", regardless of the country value:

Example

Select all employees that either:
are from Spain and starts with either "G", or
starts with the letter "R":

    
        SELECT * FROM Employees

        WHERE Country = 'India' AND emp_name LIKE 'G%' OR emp_name LIKE 'R%';

SQL OR Operator

The WHERE clause can contain one or more OR operators.

The OR operator is used to filter records based on more than one condition, like if you want to return all employees from Odisha but also those from Delhi:

Example:

Select all employees from Odisha or Delhi:

    
        SELECT *

        FROM employees

        WHERE state = 'Odisha' OR Country = 'Delhi';

OR vs AND

The OR operator displays a record if any of the conditions are TRUE.
The AND operator displays a record if all the conditions are TRUE.

At Least One Condition Must Be True

The following SQL statement selects all fields from employees where either City is "Bhubaneswar", emp_name starts with the letter "G" or Country is "Delhi":

Example

    
        SELECT * FROM Employees

        WHERE City = 'Bhubaneswar' OR emp_name LIKE 'G%' OR Country = 'India';

Combining AND and OR

You can combine the AND and OR operators.

The following SQL statement selects all Employees from Spain that starts with a "G" or an "R".

Make sure you use parenthesis to get the correct result.

Example

Select all Spanish Employees that starts with either "G" or "R":

    
        SELECT * FROM employees

        WHERE Country = 'India' AND (emp_name LIKE 'G%' OR emp_name LIKE 'R%');

Without parenthesis, the select statement will return all employees from India that starts with a "G", plus all employees that starts with an "R", regardless of the country value:

Example

Select all Employees that either:
are from Spain and starts with either "G", or
starts with the letter "R":

    
        SELECT * FROM Employees

        WHERE Country = 'Spain' AND emp_name LIKE 'G%' OR emp_name LIKE 'R%';

SQL NOT Operator

The NOT operator is used in combination with other operators to give the opposite result, also called the negative result.

In the select statement below we want to return all employees that are NOT from Spain:

Example

Select only the employees that are NOT from India:

    
        SELECT * FROM Customers

        WHERE NOT Country = 'India';

In the example above, the NOT operator is used in combination with the = operator, but it can be used in combination with other comparison and/or logical operators. See examples below.

NOT LIKE

Example

Select employees that does not start with the letter 'A':

    
        SELECT * FROM Employees

        WHERE emp_name NOT LIKE 'A%';

NOT BETWEEN

Example

Select employees with a emp_id not between 10 and 60:

    
        SELECT * FROM Employees

        WHERE emp_id NOT BETWEEN 10 AND 60;

NOT IN

Select employees that are not from Paris or London:

    
        SELECT * FROM employees

        
WHERE City NOT IN ('Paris', 'London');

NOT Greater Than

Example

Select employees with a cust_id not greater than 50:

    
        SELECT * FROM employees

        WHERE NOT emp_id > 50;

Note: There is a not-greater-then operator: !> that would give you the same result.

NOT Less Than

Example

Select employees with a emp_id not less than 50:

    
        SELECT * FROM Employees

        WHERE NOT emp_id < 50;

SQL INSERT INTO Statement

The INSERT INTO statement is used to insert new records in a table.

INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two ways:

1. Specify both the column names and the values to be inserted:

    
        INSERT INTO table_name (column1, column2, column3, ...)

        VALUES (value1, value2, value3, ...);

2. If you are adding values for all the columns of the table, you do not need to specify the column names in the SQL query. However, make sure the order of the values is in the same order as the columns in the table. Here, the INSERT INTO syntax would be as follows:

    
        INSERT INTO table_name

        VALUES (value1, value2, value3, ...);

INSERT INTO Example

The following SQL statement inserts a new record in the "Customers" table:

Example

    
        INSERT INTO Customers (emp_code, emp_name, Address, City, PostalCode, Country)

        VALUES (10001, 'Rajesh', 'Khandagiri', 'Bhubaneswar', 'Khorda', '751001', 'India');

Insert Data Only in Specified Columns

It is also possible to only insert data in specific columns.

The following SQL statement will insert a new record, but only insert data in the "emp_name", "City", and "Country" columns (emp_id will be updated automatically):

Example

    
        INSERT INTO employees (Name, City, Country)

        VALUES ('Rajesh', 'Bhubaneswar', 'India');

Insert Multiple Rows

It is also possible to insert multiple rows in one statement.

To insert multiple rows of data, we use the same INSERT INTO statement, but with multiple values:

Example

    
        INSERT INTO Customers (empcode, name, Address, City, PostalCode, Country)

        VALUES

        (10001, 'Rajesh', 'Bhubaneswar', 'Khurda', '751001', 'India'),

        (10002, 'Rajesh', 'Bhubaneswar', 'Khurda', '751001', 'India'),

        (10003, 'Rajesh', 'Bhubaneswar', 'Khurda', '751001', 'India'),

        (10004, 'Rajesh', 'Bhubaneswar', 'Khurda', '751001', 'India'),

What is a Primary Key?

The PRIMARY KEY constraint uniquely identifies each row in a table. It must contain UNIQUE values and has an implicit NOT NULL constraint.
A table in SQL is strictly restricted to have one and only one primary key, which is comprised of single or multiple fields (columns).

What is a UNIQUE constraint?

A UNIQUE constraint ensures that all values in a column are different. This provides uniqueness for the column(s) and helps identify each row uniquely. Unlike primary key, there can be multiple unique constraints defined per table. The code syntax for UNIQUE is quite similar to that of PRIMARY KEY and can be used interchangeably.

What is a Foreign Key?

A FOREIGN KEY comprises of single or collection of fields in a table that essentially refers to the PRIMARY KEY in another table. Foreign key constraint ensures referential integrity in the relation between two tables.
The table with the foreign key constraint is labeled as the child table, and the table containing the candidate key is labeled as the referenced or parent table.

What is a Join? List its different types.

The SQL Join clause is used to combine records (rows) from two or more tables in a SQL database based on a related column between the two.

(INNER) JOIN: Retrieves records that have matching values in both tables involved in the join. This is the widely used join for queries.
LEFT (OUTER) JOIN: Retrieves all the records/rows from the left and the matched records/rows from the right table.
RIGHT (OUTER) JOIN: Retrieves all the records/rows from the right and the matched records/rows from the left table.
FULL (OUTER) JOIN: Retrieves all the records where there is a match in either the left or right table.

What is a Self-Join?

A self JOIN is a case of regular join where a table is joined to itself based on some relation between its own column(s). Self-join uses the INNER JOIN or LEFT JOIN clause and a table alias is used to assign different names to the table within the query.

What is a Cross-Join?

Cross join can be defined as a cartesian product of the two tables included in the join. The table after join contains the same number of rows as in the cross-product of the number of rows in the two tables. If a WHERE clause is used in cross join then the query will work like an INNER JOIN.

What is an Index? Explain its different types.

A database index is a data structure that provides a quick lookup of data in a column or columns of a table. It enhances the speed of operations accessing data from a database table at the cost of additional writes and memory to maintain the index data structure.

Unique and Non-Unique Index: Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values. Once a unique index has been defined for a table, uniqueness is enforced whenever keys are added or changed within the index.
Non-unique indexes, on the other hand, are not used to enforce constraints on the tables with which they are associated. Instead, non-unique indexes are used solely to improve query performance by maintaining a sorted order of data values that are used frequently.
Clustered and Non-Clustered Index: Clustered indexes are indexes whose order of the rows in the database corresponds to the order of the rows in the index. This is why only one clustered index can exist in a given table, whereas, multiple non-clustered indexes can exist in the table.
The only difference between clustered and non-clustered indexes is that the database manager attempts to keep the data in the database in the same order as the corresponding keys appear in the clustered index.
Clustering indexes can improve the performance of most query operations because they provide a linear-access path to data stored in the database.

What is the difference between Clustered and Non-clustered index?

Clustered index modifies the way records are stored in a database based on the indexed column. A non-clustered index creates a separate entity within the table which references the original table.
Clustered index is used for easy and speedy retrieval of data from the database, whereas, fetching records from the non-clustered index is relatively slower.
SQL, a table can have a single clustered index whereas it can have multiple non-clustered indexes.

What is Data Integrity?

Data Integrity is the assurance of accuracy and consistency of data over its entire life-cycle and is a critical aspect of the design, implementation, and usage of any system which stores, processes, or retrieves data. It also defines integrity constraints to enforce business rules on the data when it is entered into an application or a database.

What is a Query?

A query is a request for data or information from a database table or combination of tables. A database query can be either a select query or an action query.

What is a Subquery? What are its types?

A subquery is a query within another query, also known as a nested query or inner query. It is used to restrict or enhance the data to be queried by the main query, thus restricting or enhancing the output of the main query respectively.

There are two types of subquery - Correlated and Non-Correlated.

A correlated subquery cannot be considered as an independent query, but it can refer to the column in a table listed in the FROM of the main query.
A non-correlated subquery can be considered as an independent query and the output of the subquery is substituted in the main query.

What are some common clauses used with SELECT query in SQL?

WHERE clause in SQL is used to filter records that are necessary, based on specific conditions.
ORDER BY clause in SQL is used to sort the records based on some field(s) in ascending (ASC) or descending order (DESC).
GROUP BY clause in SQL is used to group records with identical data and can be used in conjunction with some aggregation functions to produce summarized results from the database.
HAVING clause in SQL is used to filter records in combination with the GROUP BY clause. It is different from WHERE, since the WHERE clause cannot filter aggregated records.