# Select

```
SELECT [hints, ...] [ALL|DISTINCT] named_expression[, named_expression, ...]
FROM relation[, relation, ...]
[lateral_view[, lateral_view, ...]]
[WHERE boolean_expression]
[aggregation [HAVING boolean_expression]]
[ORDER BY sort_expressions]
[CLUSTER BY expressions]
[DISTRIBUTE BY expressions]
[SORT BY sort_expressions]
[WINDOW named_window[, WINDOW named_window, ...]]
[LIMIT num_rows]
named_expression:
: expression [AS alias]
relation:
| join_relation
| (table_name|query|relation) [sample] [AS alias]
: VALUES (expressions)[, (expressions), ...]
[AS (column_name[, column_name, ...])]
expressions:
: expression[, expression, ...]
sort_expressions:
: expression [ASC|DESC][, expression [ASC|DESC], ...]
```

Output data from one or more relations.

A relation refers to any source of input data. It could be the contents of an existing table (or view), the joined result of two existing tables, or a subquery (the result of another `SELECT`

statement).

(Databricks Delta) In Delta, you specify a relation either by providing a path to a table: `delta./path/to/table_name`

or a table name: `table_name`

. In addition, you can specify a time travel version using `TIMESTAMP AS OF`

, `VERSION AS OF`

, or `@`

syntax, after your table identifier. See Query an older snapshot of a table (time travel) for details.

`ALL`

- Select all matching rows from the relation. Enabled by default.
`DISTINCT`

- Select all matching rows from the relation then remove duplicate results.
`WHERE`

- Filter rows by predicate.
`HAVING`

- Filter grouped result by predicate.
`ORDER BY`

- Impose total ordering on a set of expressions. Default sort direction is ascending. You cannot use this with
`SORT BY`

,`CLUSTER BY`

, or`DISTRIBUTE BY`

. `DISTRIBUTE BY`

- Repartition rows in the relation based on a set of expressions. Rows with the same expression values will be hashed to the same worker. You cannot use this with
`ORDER BY`

or`CLUSTER BY`

. `SORT BY`

- Impose ordering on a set of expressions within each partition. Default sort direction is ascending. You cannot use this with
`ORDER BY`

or`CLUSTER BY`

. `CLUSTER BY`

- Repartition rows in the relation based on a set of expressions and sort the rows in ascending order based on the expressions. In other words, this is a shorthand for
`DISTRIBUTE BY`

and`SORT BY`

where all expressions are sorted in ascending order. You cannot use this with`ORDER BY`

,`DISTRIBUTE BY`

, or`SORT BY`

. `WINDOW`

- Assign an identifier to a window specification. See Window Functions.
`LIMIT`

- Limit the number of rows returned.
`VALUES`

- Explicitly specify values instead of reading them from a relation.

Examples:

```
SELECT * FROM boxes
SELECT width, length FROM boxes WHERE height=3
SELECT DISTINCT width, length FROM boxes WHERE height=3 LIMIT 2
SELECT * FROM VALUES (1, 2, 3) AS (width, length, height)
SELECT * FROM VALUES (1, 2, 3), (2, 3, 4) AS (width, length, height)
SELECT * FROM boxes ORDER BY width
SELECT * FROM boxes DISTRIBUTE BY width SORT BY width
SELECT * FROM boxes CLUSTER BY length
```

## Sampling

```
sample:
| TABLESAMPLE ((integer_expression | decimal_expression) PERCENT)
: TABLESAMPLE (integer_expression ROWS)
```

Sample the input data. This can be expressed in terms of either a percentage (must be between 0 and 100) or a fixed number of input rows.

Examples:

```
SELECT * FROM boxes TABLESAMPLE (3 ROWS)
SELECT * FROM boxes TABLESAMPLE (25 PERCENT)
```

## Joins

```
join_relation:
| relation join_type JOIN relation (ON boolean_expression | USING (column_name[, column_name, ...]))
: relation NATURAL join_type JOIN relation
join_type:
| INNER
| (LEFT|RIGHT) SEMI
| (LEFT|RIGHT|FULL) [OUTER]
: [LEFT] ANTI
```

`INNER JOIN`

- Select all rows from both relations where there is match.
`OUTER JOIN`

- Select all rows from both relations, filling with null values on the side that does not have a match.
`SEMI JOIN`

- Select only rows from the side of the
`SEMI JOIN`

where there is a match. If one row matches multiple rows, only the first match is returned. `LEFT ANTI JOIN`

- Select only rows from the left side that match no rows on the right side.

Examples:

```
SELECT * FROM boxes INNER JOIN rectangles ON boxes.width = rectangles.width
SELECT * FROM boxes FULL OUTER JOIN rectangles USING (width, length)
SELECT * FROM boxes NATURAL JOIN rectangles
```

## Lateral View

```
lateral_view:
: LATERAL VIEW [OUTER] function_name (expressions)
table_name [AS (column_name[, column_name, ...])]
```

Generate zero or more output rows for each input row using a table-generating function. The most common built-in function used with `LATERAL VIEW`

is `explode`

.

`LATERAL VIEW OUTER`

- Generate a row with null values even when the function returned zero rows.

Examples:

```
SELECT * FROM boxes LATERAL VIEW explode(Array(1, 2, 3)) my_view
SELECT name, my_view.grade FROM students LATERAL VIEW OUTER explode(grades) my_view AS grade
```

## Aggregation

```
aggregation:
: GROUP BY expressions [(WITH ROLLUP | WITH CUBE | GROUPING SETS (expressions))]
```

Group by a set of expressions using one or more aggregate functions. Common built-in aggregate functions include count, avg, min, max, and sum.

`ROLLUP`

- Create a grouping set at each hierarchical level of the specified expressions. For instance, For instance,
`GROUP BY a, b, c WITH ROLLUP`

is equivalent to`GROUP BY a, b, c GROUPING SETS ((a, b, c), (a, b), (a), ())`

. The total number of grouping sets will be`N + 1`

, where`N`

is the number of group expressions. `CUBE`

- Create a grouping set for each possible combination of set of the specified expressions. For instance,
`GROUP BY a, b, c WITH CUBE`

is equivalent to`GROUP BY a, b, c GROUPING SETS ((a, b, c), (a, b), (b, c), (a, c), (a), (b), (c), ())`

. The total number of grouping sets will be`2^N`

, where`N`

is the number of group expressions. `GROUPING SETS`

- Perform a group by for each subset of the group expressions specified in the grouping sets. For instance,
`GROUP BY x, y GROUPING SETS (x, y)`

is equivalent to the result of`GROUP BY x`

unioned with that of`GROUP BY y`

.

Examples:

```
SELECT height, COUNT(*) AS num_rows FROM boxes GROUP BY height
SELECT width, AVG(length) AS average_length FROM boxes GROUP BY width
SELECT width, length, height FROM boxes GROUP BY width, length, height WITH ROLLUP
SELECT width, length, avg(height) FROM boxes GROUP BY width, length GROUPING SETS (width, length)
```

## Window Functions

```
window_expression:
: expression OVER window_spec
named_window:
: window_identifier AS window_spec
window_spec:
| window_identifier
: ((PARTITION|DISTRIBUTE) BY expressions
[(ORDER|SORT) BY sort_expressions] [window_frame])
window_frame:
| (RANGE|ROWS) frame_bound
: (RANGE|ROWS) BETWEEN frame_bound AND frame_bound
frame_bound:
| CURRENT ROW
| UNBOUNDED (PRECEDING|FOLLOWING)
: expression (PRECEDING|FOLLOWING)
```

Compute a result over a range of input rows. A windowed expression is specified using the `OVER`

keyword, which is followed by either an identifier to the window (defined using the `WINDOW`

keyword) or the specification of a window.

`PARTITION BY`

- Specify which rows will be in the same partition, aliased by
`DISTRIBUTE BY`

. `ORDER BY`

- Specify how rows within a window partition are ordered, aliased by
`SORT BY`

. `RANGE bound`

- Express the size of the window in terms of a value range for the expression.
`ROWS bound`

- Express the size of the window in terms of the number of rows before and/or after the current row.
`CURRENT ROW`

- Use the current row as a bound.
`UNBOUNDED`

- Use negative infinity as the lower bound or infinity as the upper bound.
`PRECEDING`

- If used with a
`RANGE`

bound, this defines the lower bound of the value range. If used with a`ROWS`

bound, this determines the number of rows before the current row to keep in the window. `FOLLOWING`

- If used with a
`RANGE`

bound, this defines the upper bound of the value range. If used with a`ROWS`

bound, this determines the number of rows after the current row to keep in the window.

## Hints

```
hints:
: /*+ hint[, hint, ...] */
hint:
: hintName [(expression[, expression, ...])]
```

Hints can be used to help Spark execute a query better. For example, you can hint that a table is small enough to be broadcast, which would speed up joins.
You add one or more hints to a `SELECT`

statement inside /*+ ... */ comment blocks. Multiple hints can be specified inside the same comment block, in which case the hints are separated by commas, and there can be multiple such comment blocks. A hint has a name (for example, `BROADCAST`

) and accepts 0 or more parameters.

Examples:

```
SELECT /*+ BROADCAST(customers) */ * FROM customers, orders WHERE o_custId = c_custId
SELECT /*+ SKEW('orders') */ * FROM customers, orders WHERE o_custId = c_custId
SELECT /*+ SKEW('orders'), BROADCAST(demographic) */ * FROM orders, customers, demographic WHERE o_custId = c_custId AND c_demoId = d_demoId
```

See also Skew Join Optimization for more information about the `SKEW`

hint.