User Guide: Add documentation for subquery syntax (#3132)

* Add user guide section on subquery support

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

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

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

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

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

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

* prettier

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: andygrove

docs/source/user-guide/sql/index.rst

@@ -23,6 +23,7 @@ SQL Reference
 
    sql_status
    select
+   subqueries
    ddl
    aggregate_functions
    scalar_functions

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

@@ -0,0 +1,98 @@
+<!---
+  Licensed to the Apache Software Foundation (ASF) under one
+  or more contributor license agreements.  See the NOTICE file
+  distributed with this work for additional information
+  regarding copyright ownership.  The ASF licenses this file
+  to you under the Apache License, Version 2.0 (the
+  "License"); you may not use this file except in compliance
+  with the License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing,
+  software distributed under the License is distributed on an
+  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  KIND, either express or implied.  See the License for the
+  specific language governing permissions and limitations
+  under the License.
+-->
+
+# 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.
+```