With the UNION clause, each query involved must output the same number of columns, and they must be UNION compatible.

A function returns one value and has only output parameters.

A correlated subquery is where the outer query depends on data from the inner query.

Outer join is the same as equi-join, except one of the duplicate columns is eliminated in the result table.

The most frequently used relational operation, which brings together data from two or more related tables into one resultant table, is called an equi-join.