# Subqueries

DataFusion supports `EXISTS`

, `NOT EXISTS`

, `IN`

, `NOT IN`

and Scalar Subqueries.

The examples below are based on the following table.

`❯ select * from x;`

+----------+----------+

| column_1 | column_2 |

+----------+----------+

| 1 | 2 |

+----------+----------+

## EXISTS

The `EXISTS`

syntax can be used to find all rows in a relation where a correlated subquery produces one or more matches for that row. Only correlated subqueries are supported.

`❯ select * from x y where exists (select * from x where x.column_1 = y.column_1);`

+----------+----------+

| column_1 | column_2 |

+----------+----------+

| 1 | 2 |

+----------+----------+

1 row in set.

## NOT EXISTS

The `NOT EXISTS`

syntax can be used to find all rows in a relation where a correlated subquery produces zero matches for that row. Only correlated subqueries are supported.

`❯ select * from x y where not exists (select * from x where x.column_1 = y.column_1);`

0 rows in set.

## IN

The `IN`

syntax can be used to find all rows in a relation where a given expression's value can be found in the results of a correlated subquery.

`❯ select * from x where column_1 in (select column_1 from x);`

+----------+----------+

| column_1 | column_2 |

+----------+----------+

| 1 | 2 |

+----------+----------+

1 row in set.

## NOT IN

The `NOT IN`

syntax can be used to find all rows in a relation where a given expression's value can not be found in the
results of a correlated subquery.

`❯ select * from x where column_1 not in (select column_1 from x);`

0 rows in set.

## Scalar Subquery

A scalar subquery can be used to produce a single value that can be used in many different contexts in a query. Here is an example of a filter using a scalar subquery. Only correlated subqueries are supported.

`❯ select * from x y where column_1 < (select sum(column_2) from x where x.column_1 = y.column_1);`

+----------+----------+

| column_1 | column_2 |

+----------+----------+

| 1 | 2 |

+----------+----------+

1 row in set.

Large portions of this page is copied from the Apache Datafusion documentation on January 26th 2024 - where there have been customisations to match Hypi's deployment this has been noted. Apache Datafusion and the Apache name are the property of the Apache Foundation and licensed under the Apache V2 license .