This content originally appeared on DEV Community and was authored by Ricardo Tovar
Subqueries are queries that come within another larger query. They are useful to obtain specific information that will later be used in the main query.
Type
- Single row subquery:
Return a single value.
- Multirow subquery
Return multiple rows
- Correlated subquery
It depends on a column in the main query, meaning it is executed once for each row in the outer query.
- Nested subquery
A subquery that contains another subquery within it.
- Scalar subquery
Returns a single value(similar to single row subquery) but can be used as a column in the SELECT.
A subquery can appear in 3 places of your query:
- SELECT
- FROM OR INNER
- WHERE
And each one of these parts have a set of rules .
For example :
- A subquery in SELECT should return a single value like:
SELECT
nombre,
(SELECT MAX(salary) FROM Employees) AS max_salary
FROM Employees;
A subquery in FROM OR INNER should return a set of rows
A subquery in WHERE should return a column with many rows like:
SELECT NAME FROM EMPLOYEES WHERE DEPARTMENT_ID = (SELECT ID FROM DEPARTMENTS WHERE DEPARTMENT = 'IT');
This content originally appeared on DEV Community and was authored by Ricardo Tovar
Ricardo Tovar | Sciencx (2025-01-03T01:29:10+00:00) Subqueries. Retrieved from https://www.scien.cx/2025/01/03/subqueries/
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