docs/sql-pipe-syntax.md

---
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title: SQL Pipe Syntax
displayTitle: SQL Pipe Syntax
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### Syntax

#### Overview

Apache Spark supports SQL pipe syntax which allows composing queries from combinations of operators.

* Any query can have zero or more pipe operators as a suffix, delineated by the pipe character `|>`. 
* Each pipe operator starts with one or more SQL keywords followed by its own grammar as described
  in the table below.
* Most of these operators reuse existing grammar for standard SQL clauses.
* Operators can apply in any order, any number of times.

`FROM <tableName>` is now a supported standalone query which behaves the same as
`TABLE <tableName>`. This provides a convenient starting place to begin a chained pipe SQL query,
although it is possible to add one or more pipe operators to the end of any valid Spark SQL query
with the same consistent behavior as written here.

Please refer to the table at the end of this document for a full list of all supported operators
and their semantics.

#### Example

For example, this is query 13 from the TPC-H benchmark:

```sql
SELECT c_count, COUNT(*) AS custdist
FROM
  (SELECT c_custkey, COUNT(o_orderkey) c_count
   FROM customer
   LEFT OUTER JOIN orders ON c_custkey = o_custkey
     AND o_comment NOT LIKE '%unusual%packages%' GROUP BY c_custkey
  ) AS c_orders
GROUP BY c_count
ORDER BY custdist DESC, c_count DESC;
```

To write the same logic using SQL pipe operators, we express it like this:

```sql
FROM customer
|> LEFT OUTER JOIN orders ON c_custkey = o_custkey
   AND o_comment NOT LIKE '%unusual%packages%'
|> AGGREGATE COUNT(o_orderkey) c_count
   GROUP BY c_custkey
|> AGGREGATE COUNT(*) AS custdist
   GROUP BY c_count
|> ORDER BY custdist DESC, c_count DESC;
```

#### Source Tables

To start a new query using SQL pipe syntax, use the `FROM <tableName>` or `TABLE <tableName>`
clause, which creates a relation comprising all rows from the source table. Then append one or more
pipe operators to the end of this clause to perform further transformations.

#### Projections

SQL pipe syntax supports composable ways to evaluate expressions. A major advantage of these
projection features is that they support computing new expressions based on previous ones in an
incremental way. No lateral column references are needed here since each operator applies
independently on its input table, regardless of the order in which the operators appear. Each of
these computed columns then becomes visible to use with the following operator.

`SELECT` produces a new table by evaluating the provided expressions.<br>
It is possible to use `DISTINCT` and `*` as needed.<br>
This works like the outermost `SELECT` in a table subquery in regular Spark SQL.

`EXTEND` adds new columns to the input table by evaluating the provided expressions.<br>
This also preserves table aliases.<br>
This works like `SELECT *, new_column` in regular Spark SQL.

`DROP` removes columns from the input table.<br>
This is similar to `SELECT * EXCEPT (column)` in regular Spark SQL.

`SET` replaces column values from the input table.<br>
This is similar to `SELECT * REPLACE (expression AS column)` in regular Spark SQL.

`AS` forwards the input table and introduces a new alias for each row.

#### Aggregations

In general, aggregation takes place differently using SQL pipe syntax as opposed to regular Spark
SQL.

To perform full-table aggregation, use the `AGGREGATE` operator with a list of aggregate
expressions to evaluate. This returns one single row in the output table.

To perform aggregation with grouping, use the `AGGREGATE` operator with a `GROUP BY` clause.
This returns one row for each unique combination of values of the grouping expressions. The output
table contains the evaluated grouping expressions followed by the evaluated aggregate functions.
Grouping expressions support assigning aliases for purposes of referring to them in future
operators. In this way, it is not necessary to repeat entire expressions between `GROUP BY` and
`SELECT`, since `AGGREGATE` is a single operator that performs both.

#### Other Transformations

The remaining operators are used for other transformations, such as filtering, joining, sorting,
sampling, and set operations. These operators generally work in the same way as in regular Spark
SQL, as described in the table below.

### Independence and Interoperability

SQL pipe syntax works in Spark without any backwards-compatibility concerns with existing SQL
queries; it is possible to write any query using regular Spark SQL, pipe syntax, or a combination of
the two. As a consequence, the following invariants always hold:

* Each pipe operator receives an input table and operates the same way on its rows regardless of how
  it was computed.
* For any valid chain of N SQL pipe operators, any subset of the first M <= N operators also
represents a valid query.<br>
  This property can be useful for introspection and debugging, such as by selected a subset of
  lines and using the "run highlighted text" feature of SQL editors like Jupyter notebooks.
* It is possible to append pipe operators to any valid query written in regular Spark SQL.<br>
  The canonical way of starting pipe syntax queries is with the `FROM <tableName>` clause.<br>
  Note that this is a valid standalone query and may be replaced with any other Spark SQL query
  without loss of generality.
* Table subqueries can be written using either regular Spark SQL syntax or pipe syntax.<br>
  They may appear inside enclosing queries written in either syntax.
* Other Spark SQL statements such as views and DDL and DML commands may include queries written
  using either syntax.

### Supported Operators

| Operator                                          | Output rows                                                                                                         |
|---------------------------------------------------|---------------------------------------------------------------------------------------------------------------------|
| [FROM](#from-or-table) or [TABLE](#from-or-table) | Returns all the output rows from the source table unmodified.                                                       |
| [SELECT](#select)                                 | Evaluates the provided expressions over each of the rows of the input table.                                        |
| [EXTEND](#extend)                                 | Appends new columns to the input table by evaluating the specified expressions over each of the input rows.         |
| [SET](#set)                                       | Updates columns of the input table by replacing them with the result of evaluating the provided expressions.        |
| [DROP](#drop)                                     | Drops columns of the input table by name.                                                                           |
| [AS](#as)                                         | Retains the same rows and column names of the input table but with a new table alias.                               |
| [WHERE](#where)                                   | Returns the subset of input rows passing the condition.                                                             |
| [LIMIT](#limit)                                   | Returns the specified number of input rows, preserving ordering (if any).                                           |
| [AGGREGATE](#aggregate)                           | Performs aggregation with or without grouping.                                                                      |
| [JOIN](#join)                                     | Joins rows from both inputs, returning a filtered cross-product of the input table and the table argument.          |
| [ORDER BY](#order-by)                             | Returns the input rows after sorting as indicated.                                                                  |
| [UNION ALL](#union-intersect-except)              | Performs the union or other set operation over the combined rows from the input table plus other table argument(s). |
| [TABLESAMPLE](#tablesample)                       | Returns the subset of rows chosen by the provided sampling algorithm.                                               |
| [PIVOT](#pivot)                                   | Returns a new table with the input rows pivoted to become columns.                                                  |
| [UNPIVOT](#unpivot)                               | Returns a new table with the input columns pivoted to become rows.                                                  |

This table lists each of the supported pipe operators and describes the output rows they produce.
Note that each operator accepts an input relation comprising the rows generated by the query
preceding the `|>` symbol.

#### FROM or TABLE

```sql
FROM <tableName>
```

```sql
TABLE <tableName>
```

Returns all the output rows from the source table unmodified.

For example:

```sql
CREATE TABLE t AS VALUES (1, 2), (3, 4) AS t(a, b);
TABLE t;

+---+---+
|  a|  b|
+---+---+
|  1|  2|
|  3|  4|
+---+---+
```

#### SELECT

```sql
|> SELECT <expr> [[AS] alias], ...
```

Evaluates the provided expressions over each of the rows of the input table.                                                                                                                                                    

In general, this operator is not always required with SQL pipe syntax. It is possible to use it at
or near the end of a query to evaluate expressions or specify a list of output columns.

Since the final query result always comprises the columns returned from the last pipe operator,
when this `SELECT` operator does not appear, the output includes all columns from the full row.
This behavior is similar to `SELECT *` in standard SQL syntax.

It is possible to use `DISTINCT` and `*` as needed.<br>
This works like the outermost `SELECT` in a table subquery in regular Spark SQL.

Window functions are supported in the `SELECT` list as well. To use them, the `OVER` clause must be
provided. You may provide the window specification in the `WINDOW` clause.

Aggregate functions are not supported in this operator. To perform aggregation, use the `AGGREGATE`
operator instead.

For example:

```sql
CREATE TABLE t AS VALUES (0), (1) AS t(col);

FROM t
|> SELECT col * 2 AS result;

+------+
|result|
+------+
|     0|
|     2|
+------+
```

#### EXTEND

```sql
|> EXTEND <expr> [[AS] alias], ...
```

Appends new columns to the input table by evaluating the specified expressions over each of the
input rows.

After an `EXTEND` operation, top-level column names are updated but table aliases still refer to the
original row values (such as an inner join between two tables `lhs` and `rhs` with a subsequent
`EXTEND` and then `SELECT lhs.col, rhs.col`).

For example:

```sql
VALUES (0), (1) tab(col)
|> EXTEND col * 2 AS result;

+---+------+
|col|result|
+---+------+
|  0|     0|
|  1|     2|
+---+------+
```

#### SET

```sql
|> SET <column> = <expression>, ...
```

Updates columns of the input table by replacing them with the result of evaluating the provided
expressions. Each such column reference must appear in the input table exactly once.

This is similar to `SELECT * EXCEPT (column), <expression> AS column` in regular Spark SQL.

It is possible to perform multiple assignments in a single `SET` clause. Each assignment may refer
to the result of previous assignments.

After an assignment, top-level column names are updated but table aliases still refer to the
original row values (such as an inner join between two tables `lhs` and `rhs` with a subsequent
`SET` and then `SELECT lhs.col, rhs.col`).

For example:

```sql
VALUES (0), (1) tab(col)
|> SET col = col * 2;

+---+
|col|
+---+
|  0|
|  2|
+---+

VALUES (0), (1) tab(col)
|> SET col = col * 2;

+---+
|col|
+---+
|  0|
|  2|
+---+
```

#### DROP

```sql
|> DROP <column>, ...
```

Drops columns of the input table by name. Each such column reference must appear in the input table
exactly once.

This is similar to `SELECT * EXCEPT (column)` in regular Spark SQL.

After a `DROP` operation, top-level column names are updated but table aliases still refer to the
original row values (such as an inner join between two tables `lhs` and `rhs` with a subsequent
`DROP` and then `SELECT lhs.col, rhs.col`).

For example:

```sql
VALUES (0, 1) tab(col1, col2)
|> DROP col1;

+----+
|col2|
+----+
|   1|
+----+
```

#### AS

```sql
|> AS <alias>
```

Retains the same rows and column names of the input table but with a new table alias.

This operator is useful for introducing a new alias for the input table, which can then be referred
to in subsequent operators. Any existing alias for the table is replaced by the new alias.

It is useful to use this operator after adding new columns with `SELECT` or `EXTEND` or after
performing aggregation with `AGGREGATE`. This simplifies the process of referring to the columns
from subsequent `JOIN` operators and allows for more readable queries.

For example:

```sql
VALUES (0, 1) tab(col1, col2)
|> AS new_tab
|> SELECT col1 + col2 FROM new_tab;

+-----------+
|col1 + col2|
+-----------+
|          1|
+-----------+
```

#### WHERE

```sql
|> WHERE <condition>
```

Returns the subset of input rows passing the condition.

Since this operator may appear anywhere, no separate `HAVING` or `QUALIFY` syntax is needed.

For example:

```sql
VALUES (0), (1) tab(col)
|> WHERE col = 1;

+---+
|col|
+---+
|  1|
+---+
```

#### LIMIT

```sql
|> [LIMIT <n>] [OFFSET <m>]
```

Returns the specified number of input rows, preserving ordering (if any).

`LIMIT` and `OFFSET` are supported together. The `LIMIT` clause can also be used without the
`OFFSET` clause, and the `OFFSET` clause can be used without the `LIMIT` clause.

For example:

```sql
VALUES (0), (0) tab(col)
|> LIMIT 1;

+---+
|col|
+---+
|  0|
+---+
```

#### AGGREGATE

```sql
-- Full-table aggregation
|> AGGREGATE <agg_expr> [[AS] alias], ...

-- Aggregation with grouping
|> AGGREGATE [<agg_expr> [[AS] alias], ...] GROUP BY <grouping_expr> [AS alias], ...
```

Performs aggregation across grouped rows or across the entire input table.

If no `GROUP BY` clause is present, this performs full-table aggregation, returning one result row
with a column for each aggregate expression. Othwrise, this performs aggregation with grouping,
returning one row per group. Aliases can be assigned directly on grouping expressions.

The output column list of this operator includes the grouping columns first (if any), and then the
aggregate columns afterward. 

Each `<agg_expr>` expression can include standard aggregate function(s) like `COUNT`, `SUM`, `AVG`,
`MIN`, or any other aggregate function(s) that Spark SQL supports. Additional expressions may appear
below or above the aggregate function(s), such as `MIN(FLOOR(col)) + 1`. Each `<agg_expr>`
expression must contain at least one aggregate function (or otherwise the query returns an error).
Each `<agg_expr>` expression may include a column alias with `AS <alias>`, and may also
include a `DISTINCT` keyword to remove duplicate values before applying the aggregate function (for
example, `COUNT(DISTINCT col)`).

If present, the `GROUP BY` clause can include any number of grouping expressions, and each
`<agg_expr>` expression will evaluate over each unique combination of values of the grouping
expressions. The output table contains the evaluated grouping expressions followed by the evaluated
aggregate functions. The `GROUP BY` expressions may include one-based ordinals. Unlike regular SQL
in which such ordinals refer to the expressions in the accompanying `SELECT` clause, in SQL pipe
syntax, they refer to the columns of the relation produced by the preceding operator instead. For
example, in `TABLE t |> AGGREGATE COUNT(*) GROUP BY 2`, we refer to the second column of the input
table `t`.

There is no need to repeat entire expressions between `GROUP BY` and `SELECT`, since the `AGGREGATE` 
operator automatically includes the evaluated grouping expressions in its output. By the same token,
after an `AGGREGATE` operator, it is often unnecessary to issue a following `SELECT` operator, since
`AGGREGATE` returns both the grouping columns and the aggregate columns in a single step.

For example:

```sql
-- Full-table aggregation
VALUES (0), (1) tab(col)
|> AGGREGATE COUNT(col) AS count;

+-----+
|count|
+-----+
|    2|
+-----+

-- Aggregation with grouping
VALUES (0, 1), (0, 2) tab(col1, col2)
|> AGGREGATE COUNT(col2) AS count GROUP BY col1;

+----+-----+
|col1|count|
+----+-----+
|   0|    2|
+----+-----+
```

#### JOIN

```sql
|> [LEFT | RIGHT | FULL | CROSS | SEMI | ANTI | NATURAL | LATERAL] JOIN <table> [ON <condition> | USING(col, ...)]
```

Joins rows from both inputs, returning a filtered cross-product of the pipe input table and the
table expression following the JOIN keyword. This behaves a similar manner as the `JOIN` clause in
regular SQL where the pipe operator input table becomes the left side of the join and the table
argument becomes the right side of the join.

Standard join modifiers like `LEFT`, `RIGHT`, and `FULL` are supported before the `JOIN` keyword.

The join predicate may need to refer to columns from both inputs to the join. In this case, it may
be necessary to use table aliases to differentiate between columns in the event that both inputs
have columns with the same names. The `AS` operator can be useful here to introduce a new alias for
the pipe input table that becomes the left side of the join. Use standard syntax to assign an alias
to the table argument that becomes the right side of the join, if needed.

For example:

```sql
SELECT 0 AS a, 1 AS b
|> AS lhs
|> JOIN VALUES (0, 2) rhs(a, b) ON (lhs.a = rhs.a);

+---+---+---+---+
|  a|  b|  c|  d|
+---+---+---+---+
|  0|  1|  0|  2|
+---+---+---+---+

VALUES ('apples', 3), ('bananas', 4) t(item, sales)
|> AS produce_sales
|> LEFT JOIN
     (SELECT "apples" AS item, 123 AS id) AS produce_data
     USING (item)
|> SELECT produce_sales.item, sales, id;

/*---------+-------+------+
 | item    | sales | id   |
 +---------+-------+------+
 | apples  | 3     | 123  |
 | bananas | 4     | NULL |
 +---------+-------+------*/
```

#### ORDER BY

```sql
|> ORDER BY <expr> [ASC | DESC], ...
```

Returns the input rows after sorting as indicated. Standard modifiers are supported including NULLS
FIRST/LAST.

For example:

```sql
VALUES (0), (1) tab(col)
|> ORDER BY col DESC;

+---+
|col|
+---+
|  1|
|  0|
+---+
```

#### UNION, INTERSECT, EXCEPT

```sql
|> {UNION | INTERSECT | EXCEPT} {ALL | DISTINCT} (<query>)
```

Performs the union or other set operation over the combined rows from the input table or subquery.

For example:

```sql
VALUES (0), (1) tab(a, b)
|> UNION ALL VALUES (2), (3) tab(c, d);

+---+----+
|  a|   b|
+---+----+
|  0|   1|
|  2|   3|
+---+----+
```

#### TABLESAMPLE

```sql
|> TABLESAMPLE <method>(<size> {ROWS | PERCENT})
```

Returns the subset of rows chosen by the provided sampling algorithm.

For example:

```sql
VALUES (0), (0), (0), (0) tab(col)
|> TABLESAMPLE (1 ROWS);

+---+
|col|
+---+
|  0|
+---+

VALUES (0), (0) tab(col)
|> TABLESAMPLE (100 PERCENT);

+---+
|col|
+---+
|  0|
|  0|
+---+
```

#### PIVOT

```sql
|> PIVOT (agg_expr FOR col IN (val1, ...))
```

Returns a new table with the input rows pivoted to become columns.

For example:

```sql
VALUES
  ("dotNET", 2012, 10000),
  ("Java", 2012, 20000),
  ("dotNET", 2012, 5000),
  ("dotNET", 2013, 48000),
  ("Java", 2013, 30000)
  courseSales(course, year, earnings)
|> PIVOT (
     SUM(earnings)
     FOR COURSE IN ('dotNET', 'Java')
  )

+----+------+------+
|year|dotNET|  Java|
+----+------+------+
|2012| 15000| 20000|
|2013| 48000| 30000|
+----+------+------+
```

#### UNPIVOT

```sql
|> UNPIVOT (value_col FOR key_col IN (col1, ...))
```

Returns a new table with the input columns pivoted to become rows.

For example:

```sql
VALUES
  ("dotNET", 2012, 10000),
  ("Java", 2012, 20000),
  ("dotNET", 2012, 5000),
  ("dotNET", 2013, 48000),
  ("Java", 2013, 30000)
  courseSales(course, year, earnings)
|> UNPIVOT (
  earningsYear FOR `year` IN (`2012`, `2013`, `2014`)

+--------+------+--------+
|  course|  year|earnings|
+--------+------+--------+
|    Java|  2012|   20000|
|    Java|  2013|   30000|
|  dotNET|  2012|   15000|
|  dotNET|  2013|   48000|
|  dotNET|  2014|   22500|
+--------+------+--------+
```