SELECT syntax

The queries in DataFusion scan data from tables and return 0 or more rows. In this documentation we describe the SQL syntax in DataFusion.

DataFusion supports the following syntax for queries:

[ WITH with_query [, …] ]
SELECT [ ALL | DISTINCT ] select_expr [, …]
[ FROM from_item [, …] ]
[ JOIN join_item [, …] ]
[ WHERE condition ]
[ GROUP BY grouping_element [, …] ]
[ HAVING condition]
[ UNION [ ALL | select ]
[ ORDER BY expression [ ASC | DESC ][, …] ]
[ LIMIT count ]

WITH clause

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

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

SELECT clause

Example:

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.

SELECT DISTINCT person, age FROM employees

FROM clause

Example:

SELECT t.a FROM table AS t

WHERE clause

Example:

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:

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.

❯ 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.

❯ 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.

❯ 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.

❯ 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.

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

GROUP BY clause

Example:

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

HAVING clause

Example:

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

UNION clause

Example:

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:

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:

SELECT age, person FROM table
LIMIT 10