SQL
Subquery Basics: Scalar and Single Row Subqueries
A query inside a query. Learn where a subquery may appear, what each position requires it to return, and the error a scalar subquery raises when it re...
Subqueries
Scalar, multi row and correlated subqueries, EXISTS, and when to prefer a join.
4 notes
Notes
SQL
Multi Row Subqueries: IN, ANY and ALL
Subqueries that return a list of values, and the three operators that consume them - including the NULL trap that makes NOT IN return nothing.
SQL
Correlated Subqueries, EXISTS and NOT EXISTS
Subqueries that reference the outer row and run once per row. Learn EXISTS and NOT EXISTS, why they are NULL safe, and when correlation costs too much...
SQL
Subqueries in SELECT, FROM and WHERE: Join vs Subquery vs EXISTS
The same question answered four ways, with a straight comparison of readability, NULL safety and cost - and a rule for choosing between them.