Subquery or Inner query or Nested query is a query within another SQL query and embedded within the WHERE clause. subquery is used to return data that will be used in the main query as a condition to …

A subquery in WITH clause returns a temporary relation that can be used by an associated query [WITH -- not supported by MySQL 5.6, 5.7; work on MySQL 8.0 (GCP and CS server) and XAMPP MariaDB]

On the Query tab for the subquery, click the Select link next to the desired output field. Note that only a single output field can be chosen in the subquery: a Select link is shown instead of the checkboxes.

If a subquery (inner query) returns a null value to the outer query, the outer query will not return any rows when using certain comparison operators in a WHERE clause.

Use a scalar subquery in the where clause • Exercise: write a query that lists all agents from Poland who have a security clearance equal to the maximum security clearance of agents from the city of Boston.

Subqueries in WHERE/HAVING Can use subquery that returns single value; same as in SELECT Additional predicates: EXISTS / NOT EXISTS IN / NOT IN ANY / ALL Find all employees who earn …

Scoping rules: an attribute in a subquery belongs to one of the tuple variables in that subquery's FROM clause, or to the immediately surrounding subquery, and so on.