SQL Subqueries

Subquery is a query within a query.

SQL Subquery

Subquery is a query is written inside a query. The result of inner query is used in execution of outer query. You can create subqueries within your SQL statements. These subqueries can reside in the WHERE clause, the FROM clause, or the SELECT clause.

We will use STUDENT, COURSE, STUDENT_COURSE tables for understanding nested queries.

STUDENT

S_ID	S_NAME	S_ADDRESS	S_PHONE	S_AGE
S1	RAM	DELHI	9455123451	18
S2	RAMESH	GURGAON	9652431543	18
S3	SUJIT	ROHTAK	9156253131	20
S4	SURESH	DELHI	9156768971	18

COURSE

C_ID	C_NAME
C1	DSA
C2	Programming
C3	DBMS

STUDENT_COURSE

S_ID	C_ID
S1	C1
S1	C3
S2	C1
S3	C2
S4	C2
S4	C3

There are mainly two types of nested queries:

Nested Sub Queries

In independent nested queries, query execution starts from innermost query to outermost queries. The execution of inner query is independent of outer query, but the result of inner query is used in execution of outer query. Various operators like IN, NOT IN, ANY, ALL etc are used in writing independent nested queries.

IN: If we want to find out S_ID who are enrolled in C_NAME ‘DSA’ or ‘DBMS’, we can write it with the help of independent nested query and IN operator. From COURSE table, we can find out C_ID for C_NAME ‘DSA’ or DBMS’ and we can use these C_IDs for finding S_IDs from STUDENT_COURSE TABLE.

STEP 1: Finding C_ID for C_NAME =’DSA’ or ‘DBMS’

Select C_ID from COURSE where C_NAME = ‘DSA’ or C_NAME = ‘DBMS’;

STEP 2: Using C_ID of step 1 for finding S_ID

Select S_ID from STUDENT_COURSE where C_ID IN
(SELECT C_ID from COURSE where C_NAME = ‘DSA’ or C_NAME=’DBMS’);

The inner query will return a set with members C1 and C3 and outer query will return those S_IDs for which C_ID is equal to any member of set (C1 and C3 in this case). So, it will return S1, S2 and S4.

Note: If we want to find out names of STUDENTs who have either enrolled in ‘DSA’ or ‘DBMS’, it can be done as:

Select S_NAME from STUDENT where S_ID IN
(Select S_ID from STUDENT_COURSE where C_ID IN
(SELECT C_ID from COURSE where C_NAME=’DSA’ or C_NAME=’DBMS’));

NOT IN: If we want to find out S_IDs of STUDENTs who have neither enrolled in ‘DSA’ nor in ‘DBMS’, it can be done as:

Select S_ID from STUDENT where S_ID NOT IN
(Select S_ID from STUDENT_COURSE where C_ID IN
(SELECT C_ID from COURSE where C_NAME=’DSA’ or C_NAME=’DBMS’));

The innermost query will return a set with members C1 and C3. Second inner query will return those S_IDs for which C_ID is equal to any member of set (C1 and C3 in this case) which are S1, S2 and S4. The outermost query will return those S_IDs where S_ID is not a member of set (S1, S2 and S4). So it will return S3.

Co-related Sub Queries

In co-related nested queries, the output of inner query depends on the row which is being currently executed in outer query. e.g.; If we want to find out S_NAME of STUDENTs who are enrolled in C_ID ‘C1’, it can be done with the help of co-related nested query as:

Select S_NAME from STUDENT S where EXISTS
( select * from STUDENT_COURSE SC where S.S_ID=SC.S_ID and SC.C_ID=’C1’);

For each row of STUDENT S, it will find the rows from STUDENT_COURSE where S.S_ID = SC.S_IDand SC.C_ID=’C1’. If for a S_ID from STUDENT S, atleast a row exists in STUDENT_COURSE SC with C_ID=’C1’, then inner query will return true and corresponding S_ID will be returned as output.

This SQL Server tutorial explains how to use subqueries in SQL Server (Transact-SQL) with syntax and examples.

WHERE clause

Most often, the subquery will be found in the WHERE clause. These subqueries are also called nested subqueries.

For example:

SELECT p.product_id, p.product_name
FROM products p
WHERE p.product_id IN
   (SELECT inv.product_id
    FROM inventory inv
    WHERE inv.quantity > 10);

The subquery portion of the SELECT statement above is:

(SELECT inv.product_id
 FROM inventory inv
 WHERE inv.quantity > 10);

This subquery allows you to find all product_id values from the inventory table that have a quantity greater than 10. The subquery is then used to filter the results from the main query using the IN condition.

This subquery could have alternatively been written as an INNER join as follows:

SELECT p.product_id, p.product_name
FROM products p
INNER JOIN inventory inv
ON p.product_id = inv.product_id
WHERE inv.quantity > 10;

This INNER JOIN would run more efficiently than the original subquery. It is important to note, though, that not all subqueries can be rewritten using joins.

FROM clause

A subquery can also be found in the FROM clause. These are called inline views.

For example:

SELECT suppliers.supplier_name, subquery1.total_amt
FROM suppliers,
 (SELECT supplier_id, SUM(orders.amount) AS total_amt
  FROM orders
  GROUP BY supplier_id) subquery1
WHERE subquery1.supplier_id = suppliers.supplier_id;

In this example, we've created a subquery in the FROM clause as follows:

(SELECT supplier_id, SUM(orders.amount) AS total_amt
 FROM orders
 GROUP BY supplier_id) subquery1

This subquery has been aliased with the name subquery1. This will be the name used to reference this subquery or any of its fields.

SELECT clause

A subquery can also be found in the SELECT clause. These are generally used when you wish to retrieve a calculation using an aggregate function such as the SUM, COUNT, MIN, or MAX function, but you do not want the aggregate function to apply to the main query.

For example:

SELECT e1.last_name, e1.first_name,
  (SELECT MAX(salary)
   FROM employees e2
   WHERE e1.employee_id = e2.employee_id) subquery2
FROM employees e1;

In this example, we've created a subquery in the SELECT clause as follows:

(SELECT MAX(salary)
 FROM employees e2
 WHERE e1.employee_id = e2.employee_id) subquery2

The subquery has been aliased with the name subquery2. This will be the name used to reference this subquery or any of its fields.

The trick to placing a subquery in the select clause is that the subquery must return a single value. This is why an aggregate function such as the SUM, COUNT, MIN, or MAX function is commonly used in the subquery.