### Question

What happens if the tables we perform the `UNION`

operator on have duplicate rows?

### Answer

When you combine tables with `UNION`

, duplicate rows will be excluded.

To explain why this is the case, recall a Venn Diagram, which shows the relations between sets. If we perform `UNION`

on two sets of data (tables), say `A`

and `B`

, then the data returned in the result will essentially be

`A + B - (A intersect B)`

In the first part,

`A + B`

will add together all the rows of both tables, including duplicates.

The second part,

`- (A intersect B)`

will remove every duplicate, which is where `A`

and `B`

intersected.

If, however, you wanted to include duplicates, certain versions of SQL provides the `UNION ALL`

operator.