Aggregate function calls

An aggregate function summarizes the rows of a group into a single value. When an aggregate function is used with the OVER clause, it becomes a window function, which computes values over a group of rows and then returns a single result for each row.

Aggregate function call syntax

function_name(
  [ DISTINCT ]
  function_arguments
  [ { IGNORE | RESPECT } NULLS ]
  [ HAVING { MAX | MIN } having_expression ]
  [ ORDER BY key [ { ASC | DESC } ] [, ... ] ]
  [ LIMIT n ]
)
[ OVER over_clause ]

Description

Each aggregate function supports all or a subset of the aggregate function call syntax. You can use the following syntax to build an aggregate function:

DISTINCT: Each distinct value of expression is aggregated only once into the result.
IGNORE NULLS or RESPECT NULLS: If IGNORE NULLS is specified, the NULL values are excluded from the result. If RESPECT NULLS is specified, both NULL and non-NULL values can be included in the result.

If neither IGNORE NULLS nor RESPECT NULLS is specified, most functions default to IGNORE NULLS behavior but in a few cases NULL values are respected.
HAVING MAX or HAVING MIN: Restricts the set of rows that the function aggregates by a maximum or minimum value. For details, see HAVING MAX and HAVING MIN clause.
ORDER BY: Specifies the order of the values.
- For each sort key, the default sort direction is ASC.
- NULL is the minimum possible value, so NULLs appear first in ASC sorts and last in DESC sorts.
- If you're using floating point data types, see Floating point semantics on ordering and grouping.
- The ORDER BY clause is supported only for aggregate functions that depend on the order of their input. For those functions, if the ORDER BY clause is omitted, the output is nondeterministic.
- This ORDER BY clause can't be used if the OVER clause is used.
- If DISTINCT is also specified, then the sort key must be the same as expression.
LIMIT: Specifies the maximum number of expression inputs in the result.
- If the input is an ARRAY value, the limit applies to the number of input arrays, not the number of elements in the arrays. An empty array counts as 1. A NULL array isn't counted.
- If the input is a STRING value, the limit applies to the number of input strings, not the number of characters or bytes in the inputs. An empty string counts as 1. A NULL string isn't counted.
- The limit n must be a constant INT64.
OVER: If the aggregate function is also a window function, use this clause to define a window of rows around the row being evaluated. For each row, the aggregate function result is computed using the selected window of rows as input. If this clause is used, aggregate function clauses (i.e. DISTINCT) can't be used. To learn more about the OVER clause, see Window function calls.

Details

The clauses in an aggregate function call are applied in the following order:

OVER
DISTINCT
IGNORE NULLS or RESPECT NULLS
HAVING MAX or HAVING MIN
ORDER BY
LIMIT

When used in conjunction with a GROUP BY clause, the groups summarized typically have at least one row. When the associated SELECT statement has no GROUP BY clause or when certain aggregate function modifiers filter rows from the group to be summarized, it's possible that the aggregate function needs to summarize an empty group.

Restrict aggregation by a maximum or minimum value

Some aggregate functions support two optional clauses that are called HAVING MAX and HAVING MIN. These clauses restrict the set of rows that a function aggregates to rows that have a maximum or minimum value in a particular column.

HAVING MAX clause

HAVING MAX having_expression

HAVING MAX restricts the set of input rows that the function aggregates to only those with the maximum having_expression value. The maximum value is computed as the result of MAX(having_expression) across rows in the group. Only rows whose having_expression value is equal to this maximum value (using SQL equality semantics) are included in the aggregation. All other rows are ignored in the aggregation.

This clause supports all orderable data types, except for ARRAY.

Examples

In the following query, rows with the most inches of precipitation, 4, are added to a group, and then the year for one of these rows is produced. Which row is produced is nondeterministic, not random.

WITH
  Precipitation AS (
    SELECT 2009 AS year, 'spring' AS season, 3 AS inches
    UNION ALL
    SELECT 2001, 'winter', 4
    UNION ALL
    SELECT 2003, 'fall', 1
    UNION ALL
    SELECT 2002, 'spring', 4
    UNION ALL
    SELECT 2005, 'summer', 1
  )
SELECT ANY_VALUE(year HAVING MAX inches) AS any_year_with_max_inches FROM Precipitation;

/*--------------------------*
 | any_year_with_max_inches |
 +--------------------------+
 | 2001                     |
 *--------------------------*/

HAVING MIN clause

HAVING MIN having_expression

HAVING MIN restricts the set of input rows that the function aggregates to only those with the minimum having_expression value. The minimum value is computed as the result of MIN(having_expression) across rows in the group. Only rows whose having_expression value is equal to this minimum value (using SQL equality semantics) are included in the aggregation. All other rows are ignored in the aggregation.

This clause supports all orderable data types, except for ARRAY.

Examples

In the following query, rows with the fewest inches of precipitation, 1, are added to a group, and then the year for one of these rows is produced. Which row is produced is nondeterministic, not random.

WITH
  Precipitation AS (
    SELECT 2009 AS year, 'spring' AS season, 3 AS inches
    UNION ALL
    SELECT 2001, 'winter', 4
    UNION ALL
    SELECT 2003, 'fall', 1
    UNION ALL
    SELECT 2002, 'spring', 4
    UNION ALL
    SELECT 2005, 'summer', 1
  )
SELECT ANY_VALUE(year HAVING MIN inches) AS any_year_with_min_inches FROM Precipitation;

/*--------------------------*
 | any_year_with_min_inches |
 +--------------------------+
 | 2003                     |
 *--------------------------*/

Aggregate function examples

A simple aggregate function call for COUNT, MIN, and MAX looks like this:

SELECT
  COUNT(*) AS total_count,
  COUNT(fruit) AS non_null_count,
  MIN(fruit) AS min,
  MAX(fruit) AS max
FROM
  (
    SELECT NULL AS fruit
    UNION ALL
    SELECT 'apple' AS fruit
    UNION ALL
    SELECT 'pear' AS fruit
    UNION ALL
    SELECT 'orange' AS fruit
  )

/*-------------+----------------+-------+------*
 | total_count | non_null_count | min   | max  |
 +-------------+----------------+-------+------+
 | 4           | 3              | apple | pear |
 *-------------+----------------+-------+------*/

In the following example, the average of x over a specified window is returned for each row. To learn more about windows and how to use them, see Window function calls.

SELECT
  x,
  AVG(x) OVER (ORDER BY x ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) AS avg
FROM UNNEST([0, 2, 4, 4, 5]) AS x;

/*------+------*
 | x    | avg  |
 +------+------+
 | 0    | 0    |
 | 2    | 1    |
 | 4    | 3    |
 | 4    | 4    |
 | 5    | 4.5  |
 *------+------*/

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