# Set Operations

We can perform the following set operations in SQL:

* union
* intersect
* except (for difference)

```
 select * from r union/intersect/except select * from s
```

Each of these operations automatically eliminates duplicates. To retain duplicates, we must use **union all**, **intersect all** and **except all**.

Suppose a tuple occurs m times in r and n times in s, then, it occurs:

* m + n times in r union all s
* min(m,n) times in r intersect all s
* max(0, m–n) times in r except all s