docs/source/user-guide/sql/subqueries.md

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# Subqueries

DataFusion supports `EXISTS`, `NOT EXISTS`, `IN`, `NOT IN` and Scalar Subqueries.

The examples below are based on the following table.

```sql
❯ 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.

```sql
❯ 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.

```sql
❯ 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.

```sql
❯ 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.

```sql
❯ 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.

```sql
❯ 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.
```