SUB-QUERY

A MySQL subquery is a query that is nested inside another query such as SELECT, INSERT, UPDATE or DELETE. A MySQL subquery is also can be nested inside another subquery. A MySQL subquery is also called an inner query, while the query that contains the subquery is called an outer query.

Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN etc.

There are a few rules that subqueries must follow:

• Subqueries must be enclosed within parentheses.
• A subquery can have only one column in the SELECT clause unless multiple columns are in the main query for the subquery to compare its selected columns.
• An ORDER BY cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY can be used to perform the same function as the ORDER BY in a subquery.
• Subqueries that return more than one row can only be used with multiple value operators, such as the IN operator.
• The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
• A subquery cannot be immediately enclosed in a set function.
• The BETWEEN operator cannot be used with a subquery; however, the BETWEEN operator can be used within the subquery.
  Example Table For Practice:

  Employee
  Emp_id	Name	Surname	Salary	Dept_id
  1	Rajesh	Khanna	45000	2
  2	Sneha	Gupta	55000	4
  3	Sandeep	Nehte	95000	1
  4	Kirti	Patil	25000	3
  5	Nilesh	Jadhav	45000	3
  6	Dipesh	Das	35000	5
  7	Kailash	Rane	62000	1
  8	Kavita	Deshpande	48000	3
  9	Sanjay	Datt	50000	3
  10	Shruti	Hasan	78000	1

  
  

  Subqueries with the SELECT Statement:

Basic syntax:

SELECT column_name1, column_name2  FROM table1 , table2  WHERE column_name  OPERATOR (SELECT column_name1 , column_name2 FROM table1 , table2  WHERE Condition)

SQL Syntax:

SELECT *  FROM Employee  WHERE Emp_id IN  ( SELECT Emp_id  FROM Employee  WHERE Salary > 55000 ) ;

After execution:

Emp_id	Name	Surname	Salary	Dept_id
3	Sandeep	Nehte	95000	1
7	Kailash	Rane	62000	1
10	Shruti	Hasan	78000	1

Subqueries with the INSERT Statement:

INSERT INTO table_name (column1 , column2 ) SELECT * column1 , column2 FROM table1 , table2  WHERE VALUE OPERATOR

SQL Syntax: 

INSERT INTO employee_backup SELECT *  FROM employee  WHERE Emp_id IN  ( SELECT Emp_id  FROM employee )