Subquery or Inner query or Nested query is a query in a query. SQL subquery is usually added in the WHERE Clause of the SQL statement. Most of the time, a subquery is used when you know how to search for a value using a SELECT statement, but do not know the exact value in the database.
Subqueries are an alternate way of returning data from multiple tables.
Subqueries can be used with the following SQL statements along with the comparision operators like =, <, >, >=, <= etc.
SQL Subquery Example:
1) Usually, a subquery should return only one record, but sometimes it can also return multiple records when used with operators LIKE IN, NOT IN in the where clause. The query syntax would be like,
SELECT first_name, last_name, subject
WHERE games NOT IN ('Cricket', 'Football');
Subquery output would be similar to:
2) Lets consider the student_details table which we have used earlier. If you know the name of the students who are studying science subject, you can get their id's by using this query below,
SELECT id, first_name
WHERE first_name IN ('Rahul', 'Stephen');
but, if you do not know their names, then to get their id's you need to write the query in this manner,
SELECT id, first_name
WHERE first_name IN (SELECT first_name
WHERE subject= 'Science');
In the above sql statement, first the inner query is processed first and then the outer query is processed.
SQL Subquery; INSERT Statement
3) Subquery can be used with INSERT statement to add rows of data from one or more tables to another table. Lets try to group all the students who study Maths in a table 'maths_group'.
INSERT INTO maths_group(id, name)
SELECT id, first_name || ' ' || last_name
FROM student_details WHERE subject= 'Maths'
SQL Subquery; SELECT Statement
4) A subquery can be used in the SELECT statement as follows. Lets use the product and order_items table defined in the sql_joins section.
select p.product_name, p.supplier_name, (select order_id from order_items where product_id = 101) as order_id
from product p where p.product_id = 101
A query is called correlated subquery when both the inner query and the outer query are interdependent. For every row processed by the inner query, the outer query is processed as well. The inner query depends on the outer query before it can be processed.
SELECT p.product_name FROM product p
WHERE p.product_id = (SELECT o.product_id FROM order_items o
WHERE o.product_id = p.product_id);
1) You can nest as many queries you want but it is recommended not to nest more than 16 subqueries in oracle
2) If a subquery is not dependent on the outer query it is called a non-correlated subquery
3) Minimize subquery errors: Use drag and drop, copy and paste to avoid running subqueries with spelling and database typos. Watch your multiple field SELECT comma use, extra or to few getting SQL error message "Incorrect syntax".
SQL Subquery Comments
Adding SQL Subquery comments are good habit (/* your command comment */) which can save you time, clarify your previous work .. results in less SQL headaches
SQL Subquery optimization is a good habit .. more on SQL Query Fine Tuning SQL Query Optimization