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SQL SubQuery

The Sub-query in the SQL is the inner query placed or positioned inside another query, which is also known is the outer query. The inner query is embedded in the Where clause of the SQL Query.

There are certain rules for executing the Subqueries in the SQL are as follows:

  1. We can use SQL Sub-query along with SELECT Statement, UPDATE Statement, INSERT Statement and DELETE Statement in the SQL statement.
  2. When we use Subqueries in the SQL statement, the nested inner query will get executed first, then the outer query, which is the main query executed at the end.
  3. A comparison operator can also be used.
  4. The inner query is closed within the parenthesis, and the inner query is placed on the right side of the comparison operator.
  5. You cannot use the ORDER BY clause in the Sub-query but can use the GROUP BY clause.
  6. The Sub-query with FROM clause can be used, WHERE clause, and HAVING clause.

Let's understand the SQL Sub-Query with the help of examples.

Consider the already existing table, which has the following data:

Table Number 1: - D_Students

Student_IdStudent_NameFirst_SemSecond_SemThird_SemFourth_SemFifth_SemSixth_SemTotalDepartment_Id
202111Vaishnavi Patil949188859592911
202112Vaibhav Lokhande859092808582862
202113Yash Dhull908894878590893
202114Sonali Patole959092889290914
202115Axar Patel858082869284851
202116Meena Mishra787580748577783
202117Mahesh Kumbhar758075788076775
202118Sakshi Patil807874788077782
202119Sopan Bhore706875758080752
202220Prajwal Lokhande808585757880814
202221Anuja Wanare858886828485855
202222Venkatesh Iyer908987909291903
202223Anushka Sen707571748078751
202224Aakash Jain807572748580784
202225Akshay Agarwal858078889082845
202226Shwetali Bhagwat908085889080861
202227Mayuri Wagh808085808285824
202228Utkarsh Rokade858080908484845
202229Manthan Koli857584788280812
202230Mayur Jain808887909290881

Table Number 2: Department

Department_IdDepartment_Name
1Computer Engineering
2Information Technology
3Mechanical Engineering
4Automobile Engineering
5Civil Engineering
6Electrical Engineering
7Electronics and Telecommunication Engineering
8Chemical Engineering

1. Subquery with the SELECT Statement

The SELECT Statement is used with the subqueries to display the data from the tables.

The syntax of the Subquery with the SELECT statement is as follows:

SELECT * FROM Table_Name WHERE Column_Name Expression Operator (SELECT * FROM Table_Name);

Example 1: Execute a query to display the student's information where student department names are 'Computer Engineering', 'Information Technology', and 'Automobile Engineering'. 

SELECT * FROM D_Students WHERE Department_Id IN (SELECT Department_Id FROM Department WHERE Department_Name IN ('Computer Engineering', 'Information Technology','Automobile Engineering'));

In the above query example, we display the student's information where student department names are 'Computer Engineering', 'Information Technology', and 'Automobile Engineering'.

First, SELECT Department_Id FROM Department WHERE Department_Name IN ('Computer Engineering', 'Information Technology', 'Automobile Engineering'); gets executed, and the output of this inner query is

Department_Id
1
2
4

As the inner query output is calculated, the output is represented as input for the main query. The main query is executed as SELECT * FROM D_Students WHERE Department_Id IN (1, 2, 4);

The output of the above query is as follows:

SQL SubQuery
Student_IdStudent_NameFirst_SemSecond_SemThird_SemFourth_SemFifth_SemSixth_SemTotalDepartment_Id
202111Vaishnavi Patil949188859592911
202115Axar Patel858082869284851
202223Anushka Sen707571748078751
202226Shwetali Bhagwat908085889080861
202230Mayur Jain808887909290881
202112Vaibhav Lokhande859092808582862
202118Sakshi Patil807874788077782
202119Sopan Bhore706875758080752
202229Manthan Koli857584788280812
202114Sonali Patole959092889290914
202220Prajwal Lokhande808585757880814
202224Aakash Jain807572748580784
202227Mayuri Wagh808085808285824
SQL SubQuery

Example 2: Execute a query to display the student’s information where the student’s department name is ‘Computer Engineering’

SELECT * FROM D_Students WHERE Department_Id = (SELECT Department_Id FROM Department WHERE Department_Name = 'Computer Engineering');

We display the student's information where the department name is 'Computer Engineering' in the above query. The query execution is the same as explained in the above example.

The output of the above query is as follows:

Student_IdStudent_NameFirst_SemSecond_SemThird_SemFourth_SemFifth_SemSixth_SemTotalDepartment_Id
202111Vaishnavi Patil949188859592911
202115Axar Patel858082869284851
202223Anushka Sen707571748078751
202226Shwetali Bhagwat908085889080861
202230Mayur Jain808887909290881
SQL SubQuery

2. Subquery with the UPDATE Statement

The UPDATE statement is used with the subqueries to modify the data.

The Syntax of the Subquery with the UPDATE statement is as follows:

UPDATE Table_Name SET Column_Name = Values WHERE Column_Name Conditions operator (SELECT * FROM Table_Name WHERE Column_Name Conditions);

Example 1: Execute a query to modify the student's information where the student's first-semester percentage is greater than 85, and the department name is 'Computer Engineering'.

UPDATE D_Students SET DEpartment_Id = '6' WHERE First_Sem > 85 AND Department_Id = (SELECT Department_Id FROM Department WHERE Department_Name = 'Computer Engineering');

In the above query, we are modifying the student information of those students where the first-semester percentage is greater than 85, and the department name is 'Computer Engineering’.

First, SELECT Department_Id FROM Department WHERE Department_Name = 'Computer Engineering'; gets executed, and the output of this inner query is

Department_Id
1

As the inner query output is calculated, the output is represented as input for the main query. The main query is executed as UPDATE D_Students SET Department_Id = 6 WHERE First_Sem > 85 AND Department_Id = 1;

We will execute the SELECT statement to check whether the data is modified or not.

SELECT * FROM D_Students;

The output of the above query is as follows:

Student_IdStudent_NameFirst_SemSecond_SemThird_SemFourth_SemFifth_SemSixth_SemTotalDepartment_Id
202111Vaishnavi Patil949188859592916
202112Vaibhav Lokhande859092808582862
202113Yash Dhull908894878590893
202114Sonali Patole959092889290914
202115Axar Patel858082869284851
202116Meena Mishra787580748577783
202117Mahesh Kumbhar758075788076775
202118Sakshi Patil807874788077782
202119Sopan Bhore706875758080752
202220Prajwal Lokhande808585757880814
202221Anuja Wanare858886828485855
202222Venkatesh Iyer908987909291903
202223Anushka Sen707571748078751
202224Aakash Jain807572748580784
202225Akshay Agarwal858078889082845
202226Shwetali Bhagwat908085889080866
202227Mayuri Wagh808085808285824
202228Utkarsh Rokade858080908484845
202229Manthan Koli857584788280812
202230Mayur Jain808887909290881
SQL SubQuery

Example 2: Execute a query to modify the student's information where the student name starts with the letter 'A' and department names are 'Automobile Engineering' and 'Civil Engineering'.

UPDATE D_Students SET Department_Id = '7' WHERE Student_Name LIKE 'A%' AND Department_Id IN (SELECT Department_Id FROM Department WHERE Department_Name IN ('Automobile Engineering','Civil Engineering'));

In the above query, we modify those students' information where student_name starts with the letter 'A' and department names are 'Automobile Engineering' and 'Civil Engineering'. The query execution is the same as explained in the above example.

We will execute the SELECT statement to check whether the data is modified or not.

SELECT * FROM D_Students;

The output of the above query is as follows:

Student_IdStudent_NameFirst_SemSecond_SemThird_SemFourth_SemFifth_SemSixth_SemTotalDepartment_Id
202111Vaishnavi Patil949188859592916
202112Vaibhav Lokhande859092808582862
202113Yash Dhull908894878590893
202114Sonali Patole959092889290914
202115Axar Patel858082869284851
202116Meena Mishra787580748577783
202117Mahesh Kumbhar758075788076775
202118Sakshi Patil807874788077782
202119Sopan Bhore706875758080752
202220Prajwal Lokhande808585757880814
202221Anuja Wanare858886828485857
202222Venkatesh Iyer908987909291903
202223Anushka Sen707571748078751
202224Aakash Jain807572748580787
202225Akshay Agarwal858078889082847
202226Shwetali Bhagwat908085889080866
202227Mayuri Wagh808085808285824
202228Utkarsh Rokade858080908484845
202229Manthan Koli857584788280812
202230Mayur Jain808887909290881
SQL SubQuery

3. Subquery with the DELETE Statement

The DELETE statement is used with the subqueries to remove data from the table.

The Syntax of the Subquery with the DELETE statement is as follows:

DELETE FROM Table_Name WHERE Column_Name Conditions operator (SELECT * FROM Table_Name WHERE Column_Name Conditions);

Example: Execute a query to remove the student's information where department names are 'Civil Engineering’, ‘Electrical Engineering’, and ‘Electronics and Telecommunication Engineering’.

DELETE FROM D_Students WHERE Department_Id IN (SELECT Department_Id FROM Department WHERE Department_Name IN ('Civil Engineering', 'Electrical Engineering', 'Electronics and Telecommunication Engineering'));

In the above query, we removed the student's information from the D_Students table where department names are 'Civil Engineering’, ‘Electrical Engineering’, and ‘Electronics and Telecommunication Engineering’.

First, SELECT Department_Id FROM Department WHERE Department_Name IN ('Civil Engineering', 'Electrical Engineering', 'Electronics and Telecommunication Engineering'; gets executed, and the output of this inner query is

Department_Id
5
6
7

As the inner query output is calculated, the output is represented as input for the main query. The main query is executed as DELETE FROM D_Students WHERE Department_Id IN (5, 6, 7);

The main query removes the student’s information where department id is 5, 6, and 7 from the D_Students table.

We will execute the SELECT statement to check whether the data is removed or not.

SELECT * FROM D_Students;

The output of the above query is as follows:

Student_IdStudent_NameFirst_SemSecond_SemThird_SemFourth_SemFifth_SemSixth_SemTotalDepartment_Id
202112Vaibhav Lokhande859092808582862
202113Yash Dhull908894878590893
202114Sonali Patole959092889290914
202115Axar Patel858082869284851
202116Meena Mishra787580748577783
202118Sakshi Patil807874788077782
202119Sopan Bhore706875758080752
202220Prajwal Lokhande808585757880814
202222Venkatesh Iyer908987909291903
202223Anushka Sen707571748078751
202227Mayuri Wagh808085808285824
202229Manthan Koli857584788280812
202230Mayur Jain808887909290881
SQL SubQuery



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