# `UNION`, `INTERSECT`, and `EXCEPT` Operators

By using the operators `UNION`, `INTERSECT`, and `EXCEPT`, the output of more than one `SELECT` statement can be combined to form a single result set. The `<select_statement>` can’t have an `ORDER BY` or `LIMIT` clause. `ORDER BY` and `LIMIT` can be attached to a sub-expression if it’s enclosed in parentheses. Without parentheses, these clauses apply to the result of the `UNION`.

The result of these operators, by default, doesn’t contain duplicates. To return duplicates in the result, specify the `ALL` option.

Operators in the same `SELECT` statement are evaluated from left to right. Intersect binds more tightly than `UNION` or `EXCEPT`. For example, `A UNION B INTERSECT C` is read as `A UNION (B INTERSECT C)`.

## `UNION`

The `UNION` operator computes the set union of the rows returned by the `SELECT` statements. A row is in the set union if it’s in at least one of the result sets. Provide two `SELECT` statements that produce the same number of columns, and have columns with compatible data types.

**Syntax**

```sql
<select_statement>
UNION [ ALL | DISTINCT ]
<select_statement>
```

## `INTERSECT`

The `INTERSECT` operator computes the set intersection of the rows returned by the `SELECT` statements. A row is in the intersection if it appears in both result sets. With the `ALL` option, a row that has `m` duplicates in the left table and `n` duplicates in the right table appears `min(m, n)` times in the result set.

**Syntax**

```sql
<select_statement>
INTERSECT [ ALL | DISTINCT ]
<select_statement>
```

## `EXCEPT`

The `EXCEPT` operator computes the set of rows that are in the result of the left `SELECT` statement but not in the result of the right one. With `ALL`, a row that has `m` duplicates in the left table and `n` duplicates in the right table appears `max(m - n, 0)` times in the result set.

**Syntax**

```sql
<select_statement>
EXCEPT [ ALL | DISTINCT ]
<select_statement>
```