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

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# SELECT syntax

The queries in DataFusion scan data from tables and return 0 or more rows.
Please be aware that column names in queries are made lower-case, but not on the inferred schema. Accordingly, if you
want to query against a capitalized field, make sure to use double quotes. Please see this
[example](https://arrow.apache.org/datafusion/user-guide/example-usage.html) for clarification.
In this documentation we describe the SQL syntax in DataFusion.

DataFusion supports the following syntax for queries:
<code class="language-sql hljs">

[ [WITH](#with-clause) with_query [, ...] ] <br/>
[SELECT](#select-clause) [ ALL | DISTINCT ] select_expr [, ...] <br/>
[ [FROM](#from-clause) from_item [, ...] ] <br/>
[ [JOIN](#join-clause) join_item [, ...] ] <br/>
[ [WHERE](#where-clause) condition ] <br/>
[ [GROUP BY](#group-by-clause) grouping_element [, ...] ] <br/>
[ [HAVING](#having-clause) condition] <br/>
[ [UNION](#union-clause) [ ALL | select ] <br/>
[ [ORDER BY](#order-by-clause) expression [ ASC | DESC ][, ...] ] <br/>
[ [LIMIT](#limit-clause) count ] <br/>

</code>

## WITH clause

A with clause allows to give names for queries and reference them by name.

```sql
WITH x AS (SELECT a, MAX(b) AS b FROM t GROUP BY a)
SELECT a, b FROM x;
```

## SELECT clause

Example:

```sql
SELECT a, b, a + b FROM table
```

The `DISTINCT` quantifier can be added to make the query return all distinct rows.
By default `ALL` will be used, which returns all the rows.

```sql
SELECT DISTINCT person, age FROM employees
```

## FROM clause

Example:

```sql
SELECT t.a FROM table AS t
```

## WHERE clause

Example:

```sql
SELECT a FROM table WHERE a > 10
```

## JOIN clause

DataFusion supports `INNER JOIN`, `LEFT OUTER JOIN`, `RIGHT OUTER JOIN`, `FULL OUTER JOIN`, and `CROSS JOIN`.

The following examples are based on this table:

```sql
select * from x;
+----------+----------+
| column_1 | column_2 |
+----------+----------+
| 1        | 2        |
+----------+----------+
```

### INNER JOIN

The keywords `JOIN` or `INNER JOIN` define a join that only shows rows where there is a match in both tables.

```sql
❯ select * from x inner join x y ON x.column_1 = y.column_1;
+----------+----------+----------+----------+
| column_1 | column_2 | column_1 | column_2 |
+----------+----------+----------+----------+
| 1        | 2        | 1        | 2        |
+----------+----------+----------+----------+
```

### LEFT OUTER JOIN

The keywords `LEFT JOIN` or `LEFT OUTER JOIN` define a join that includes all rows from the left table even if there
is not a match in the right table. When there is no match, null values are produced for the right side of the join.

```sql
❯ select * from x left join x y ON x.column_1 = y.column_2;
+----------+----------+----------+----------+
| column_1 | column_2 | column_1 | column_2 |
+----------+----------+----------+----------+
| 1        | 2        |          |          |
+----------+----------+----------+----------+
```

### RIGHT OUTER JOIN

The keywords `RIGHT JOIN` or `RIGHT OUTER JOIN` define a join that includes all rows from the right table even if there
is not a match in the left table. When there is no match, null values are produced for the left side of the join.

```sql
❯ select * from x right join x y ON x.column_1 = y.column_2;
+----------+----------+----------+----------+
| column_1 | column_2 | column_1 | column_2 |
+----------+----------+----------+----------+
|          |          | 1        | 2        |
+----------+----------+----------+----------+
```

### FULL OUTER JOIN

The keywords `FULL JOIN` or `FULL OUTER JOIN` define a join that is effectively a union of a `LEFT OUTER JOIN` and
`RIGHT OUTER JOIN`. It will show all rows from the left and right side of the join and will produce null values on
either side of the join where there is not a match.

```sql
❯ select * from x full outer join x y ON x.column_1 = y.column_2;
+----------+----------+----------+----------+
| column_1 | column_2 | column_1 | column_2 |
+----------+----------+----------+----------+
| 1        | 2        |          |          |
|          |          | 1        | 2        |
+----------+----------+----------+----------+
```

### CROSS JOIN

A cross join produces a cartesian product that matches every row in the left side of the join with every row in the
right side of the join.

```sql
❯ select * from x cross join x y;
+----------+----------+----------+----------+
| column_1 | column_2 | column_1 | column_2 |
+----------+----------+----------+----------+
| 1        | 2        | 1        | 2        |
+----------+----------+----------+----------+
```

## GROUP BY clause

Example:

```sql
SELECT a, b, MAX(c) FROM table GROUP BY a, b
```

## HAVING clause

Example:

```sql
SELECT a, b, MAX(c) FROM table GROUP BY a, b HAVING MAX(c) > 10
```

## UNION clause

Example:

```sql
SELECT
    a,
    b,
    c
FROM table1
UNION ALL
SELECT
    a,
    b,
    c
FROM table2
```

## ORDER BY clause

Orders the results by the referenced expression. By default it uses ascending order (`ASC`).
This order can be changed to descending by adding `DESC` after the order-by expressions.

Examples:

```sql
SELECT age, person FROM table ORDER BY age;
SELECT age, person FROM table ORDER BY age DESC;
SELECT age, person FROM table ORDER BY age, person DESC;
```

## LIMIT clause

Limits the number of rows to be a maximum of `count` rows. `count` should be a non-negative integer.

Example:

```sql
SELECT age, person FROM table
LIMIT 10
```