SQL Sub Queries

By | September 13, 2020
SQL Sub Queries

In SQL if we write a query inside another query, then it would be considered as a nested, inner query, and subquery. Generally, these subqueries applied using the SQL WHERE clause.

SQL Sub Queries

Subqueries are basically used to set a condition expression, that’s why the subquery executes before the main query. The data sets returned by the subquery become the part of condition expression on which the main query act.

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Rules to perform a Subquery

There are some set of rules which we need to follow if we want to perform the subquery.

  • The subquery must be associated with the main query using WHERE, HAVING and FROM clause.
  • The subquery must return a result so it should be used with SELECT statement.
  • The Mainquery could either be a SELECT, UPDATE, INSERT or DELETE statement.
  • The subquery must be inside the main query and enclosed in parentheses.
  • The subquery must be written on the right side of the comparison operator.
  • If the subquery supposed to return more than one value then use the multiple value operators (IN, BETWEEN) in the Mainquery.
  • If the subquery returns a single row, then use single value operators such as =, <=, >=, etc. in the main query.

Subquery Syntax:

Select Syntax

SELECT column_name,…
FROM table_name_1
WHERE column_name_x expression operator
(SELECT column_name_x  from table_name_2   WHERE ... );

Insert Syntax

INSERT INTO Table_Name_1
(SELECT columns FROM Table_name_2);

Update Syntax

UPDATE Table_name_1
SET column_name = value
WHERE column_name_n IN (SELECT column_name_n  FROM Table_Name_2 condition);

Delete Syntax

DELETE FROM Table_name_1
WHERE column_name_n IN (SELECT column_name_n FROM Table_name_2 condition);

Example

For the examples consider these two sample tables.

Students

+------+--------+------+--------+-------+----------+
| id   | name   | age  | grades | marks | Trade    |
+------+--------+------+--------+-------+----------+
|    1 | Luffy  |   16 | A      |   970 | Science  |
|    2 | Naruto |   18 | A      |   960 | Humanity |
|    3 | Zoro   |   20 | A      |   940 | Commerce |
|    4 | Sanji  |   21 | B      |   899 | Humanity |
|    5 | Nami   |   17 | B      |   896 | Science  |
|    6 | Robin  |   20 | B      |   860 | Humanity |
|    7 | Goku   |   27 | B      |   860 | Humanity |
+------+--------+------+--------+-------+----------+

Library

+---------+-------------------+------------+
| Book_ID | Book_Name         | Student_ID |
+---------+-------------------+------------+
|    1124 | One Hundred years |          1 |
|    1104 | The Great Escape  |          2 |
|    1209 | Beloved           |          6 |
|    1111 | Hollow            |          4 |
|    2351 | Invisible Man     |          3 |
|    1034 | A Passage         |          2 |
|    1211 | Hero              |          6 |
|    1188 | Your Name         |          5 |
|    1211 | Hero              |          6 |
|    1000 | My Hope           |          8 |
|    1000 | Go Away           |         10 |
+---------+-------------------+------------+

Query:  Show the ID, names and trade of those students who have borrowed books from the Library.

Main Query: Show the ID, Names and Trade of students.

Sub Query: Return Students ID, who have borrowed books from the Library.

SELECT id, name, trade
FROM students
WHERE id IN (SELECT Student_ID FROM Library);

Output

+------+--------+----------+
| id   | name   | trade    |
+------+--------+----------+
|    1 | Luffy  | Science  |
|    2 | Naruto | Humanity |
|    3 | Zoro   | Commerce |
|    4 | Sanji  | Humanity |
|    5 | Nami   | Science  |
|    6 | Robin  | Humanity |
+------+--------+----------+

<Note>: The Above query can also be performed using a SQL JOIN statement while dealing with the database we often use JOIN statement instead of Subqueries because subqueries are slower than JOIN.

Summary

  • A query inside another query is known as a Subquery.
  • A subquery is also known as a nested and inner query.
  • The Subquery act as a conditional expression for the main query.
  • The query which bound the subquery is known as the Main Query.
  • The subquery must be enclosed in parentheses.
  • The subquery can be performed within the SELECT, INSERT, UPDATE and DELETE statements of the main query.
  • The subquery must return a result.
  • Subquery gets executed before the main query.

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