# Presto 316 Documentation

## 7.14. Aggregate Functions

Aggregate functions operate on a set of values to compute a single result.

Except for `count()`, `count_if()`, `max_by()`, `min_by()` and `approx_distinct()`, all of these aggregate functions ignore null values and return null for no input rows or when all values are null. For example, `sum()` returns null rather than zero and `avg()` does not include null values in the count. The `coalesce` function can be used to convert null into zero.

Some aggregate functions such as `array_agg()` produce different results depending on the order of input values. This ordering can be specified by writing an ORDER BY Clause within the aggregate function:

    array_agg(x ORDER BY y DESC)
    array_agg(x ORDER BY x, y, z)

## General Aggregate Functions

`arbitrary`(x) → [same as input]

  Returns an arbitrary non-null value of `x`, if one exists.

`array_agg`(x) → array<[same as input]>

  Returns an array created from the input `x` elements.

`avg`(x) → double

  Returns the average (arithmetic mean) of all input values.

`avg`(time interval type) → time interval type

  Returns the average interval length of all input values.

`bool_and`(boolean) → boolean

  Returns `TRUE` if every input value is `TRUE`, otherwise `FALSE`.

`bool_or`(boolean) → boolean

  Returns `TRUE` if any input value is `TRUE`, otherwise `FALSE`.

`checksum`(x) → varbinary

  Returns an order-insensitive checksum of the given values.

`count`(*) → bigint

  Returns the number of input rows.

`count`(x) → bigint

  Returns the number of non-null input values.

`count_if`(x) → bigint

  Returns the number of `TRUE` input values. This function is equivalent to `count(CASE WHEN x THEN 1 END)`.

`every`(boolean) → boolean

  This is an alias for `bool_and()`.

`geometric_mean`(x) → double

  Returns the geometric mean of all input values.

`max_by`(x, y) → [same as x]

  Returns the value of `x` associated with the maximum value of `y` over all input values.

`max_by`(x, y, n) → array<[same as x]>

  Returns `n` values of `x` associated with the `n` largest of all input values of `y` in descending order of `y`.

`min_by`(x, y) → [same as x]

  Returns the value of `x` associated with the minimum value of `y` over all input values.

`min_by`(x, y, n) → array<[same as x]>

  Returns `n` values of `x` associated with the `n` smallest of all input values of `y` in ascending order of `y`.

`max`(x) → [same as input]

  Returns the maximum value of all input values.

`max`(x, n) → array<[same as x]>

  Returns `n` largest values of all input values of `x`.

`min`(x) → [same as input]

  Returns the minimum value of all input values.

`min`(x, n) → array<[same as x]>

  Returns `n` smallest values of all input values of `x`.

`sum`(x) → [same as input]

  Returns the sum of all input values.

## Bitwise Aggregate Functions

`bitwise_and_agg`(x) → bigint

  Returns the bitwise AND of all input values in 2’s complement representation.

`bitwise_or_agg`(x) → bigint

  Returns the bitwise OR of all input values in 2’s complement representation.

## Map Aggregate Functions

`histogram`(x) -> map(K, bigint)

  Returns a map containing the count of the number of times each input value occurs.

`map_agg`(key, value) -> map(K, V)

  Returns a map created from the input `key` / `value` pairs.

`map_union`(x(K, V)) -> map(K, V)

  Returns the union of all the input maps. If a key is found in multiple input maps, that key’s value in the resulting map comes from an arbitrary input map.

`multimap_agg`(key, value) -> map(K, array(V))

  Returns a multimap created from the input `key` / `value` pairs. Each key can be associated with multiple values.

## Approximate Aggregate Functions

`approx_distinct`(x) → bigint

  Returns the approximate number of distinct input values. This function provides an approximation of `count(DISTINCT x)`. Zero is returned if all input values are null.

  This function should produce a standard error of 2.3%, which is the standard deviation of the (approximately normal) error distribution over all possible sets. It does not guarantee an upper bound on the error for any specific input set.

`approx_distinct`(x, e) → bigint

  Returns the approximate number of distinct input values. This function provides an approximation of `count(DISTINCT x)`. Zero is returned if all input values are null.

  This function should produce a standard error of no more than `e`, which is the standard deviation of the (approximately normal) error distribution over all possible sets. It does not guarantee an upper bound on the error for any specific input set. The current implementation of this function requires that `e` be in the range of `[0.0040625, 0.26000]`.

`approx_percentile`(x, percentage) → [same as x]

  Returns the approximate percentile for all input values of `x` at the given `percentage`. The value of `percentage` must be between zero and one and must be constant for all input rows.

`approx_percentile`(x, percentages) → array<[same as x]>

  Returns the approximate percentile for all input values of `x` at each of the specified percentages. Each element of the `percentages` array must be between zero and one, and the array must be constant for all input rows.

`approx_percentile`(x, w, percentage) → [same as x]

  Returns the approximate weighed percentile for all input values of `x` using the per-item weight `w` at the percentage `p`. The weight must be an integer value of at least one. It is effectively a replication count for the value `x` in the percentile set. The value of `p` must be between zero and one and must be constant for all input rows.

`approx_percentile`(x, w, percentage, accuracy) → [same as x]

  Returns the approximate weighed percentile for all input values of `x` using the per-item weight `w` at the percentage `p`, with a maximum rank error of `accuracy`. The weight must be an integer value of at least one. It is effectively a replication count for the value `x` in the percentile set. The value of `p` must be between zero and one and must be constant for all input rows. `accuracy` must be a value greater than zero and less than one, and it must be constant for all input rows.

`approx_percentile`(x, w, percentages) → array<[same as x]>

  Returns the approximate weighed percentile for all input values of `x` using the per-item weight `w` at each of the given percentages specified in the array. The weight must be an integer value of at least one. It is effectively a replication count for the value `x` in the percentile set. Each element of the array must be between zero and one, and the array must be constant for all input rows.

`approx_set`(x) → HyperLogLog

  See HyperLogLog Functions.

`merge`(x) → HyperLogLog

  See HyperLogLog Functions.

`merge`(qdigest(T)) -> qdigest(T)

  See Quantile Digest Functions.

`qdigest_agg`(x) → qdigest<[same as x]>

  See Quantile Digest Functions.

`qdigest_agg`(x, w) → qdigest<[same as x]>

  See Quantile Digest Functions.

`qdigest_agg`(x, w, accuracy) → qdigest<[same as x]>

  See Quantile Digest Functions.

`numeric_histogram`(buckets, value, weight) → map<double, double>

  Computes an approximate histogram with up to `buckets` number of buckets for all `value`s with a per-item weight of `weight`. The algorithm is based loosely on:

      Yael Ben-Haim and Elad Tom-Tov, "A streaming parallel decision tree algorithm",
      J. Machine Learning Research 11 (2010), pp. 849--872.

  `buckets` must be a `bigint`. `value` and `weight` must be numeric.

`numeric_histogram`(buckets, value) → map<double, double>

  Computes an approximate histogram with up to `buckets` number of buckets for all `value`s. This function is equivalent to the variant of `numeric_histogram()` that takes a `weight`, with a per-item weight of `1`.

## Statistical Aggregate Functions

`corr`(y, x) → double

  Returns correlation coefficient of input values.

`covar_pop`(y, x) → double

  Returns the population covariance of input values.

`covar_samp`(y, x) → double

  Returns the sample covariance of input values.

`kurtosis`(x) → double

  Returns the excess kurtosis of all input values. Unbiased estimate using the following expression:

      kurtosis(x) = n(n+1)/((n-1)(n-2)(n-3))sum[(x_i-mean)^4]/stddev(x)^4-3(n-1)^2/((n-2)(n-3))

`regr_intercept`(y, x) → double

  Returns linear regression intercept of input values. `y` is the dependent value. `x` is the independent value.

`regr_slope`(y, x) → double

  Returns linear regression slope of input values. `y` is the dependent value. `x` is the independent value.

`skewness`(x) → double

  Returns the skewness of all input values.

`stddev`(x) → double

  This is an alias for `stddev_samp()`.

`stddev_pop`(x) → double

  Returns the population standard deviation of all input values.

`stddev_samp`(x) → double

  Returns the sample standard deviation of all input values.

`variance`(x) → double

  This is an alias for `var_samp()`.

`var_pop`(x) → double

  Returns the population variance of all input values.

`var_samp`(x) → double

  Returns the sample variance of all input values.

## Lambda Aggregate Functions

`reduce_agg`(inputValue T, initialState S, inputFunction(S, T, S), combineFunction(S, S, S)) → S

  Reduces all input values into a single value. `inputFunction` will be invoked for each non-null input value. In addition to taking the input value, `inputFunction` takes the current state, initially `initialState`, and returns the new state. `combineFunction` will be invoked to combine two states into a new state. The final state is returned:

      SELECT id, reduce_agg(value, 0, (a, b) -> a + b, (a, b) -> a + b)
      FROM (
          VALUES
              (1, 3),
              (1, 4),
              (1, 5),
              (2, 6),
              (2, 7)
      ) AS t(id, value)
      GROUP BY id;
      -- (1, 12)
      -- (2, 13)

      SELECT id, reduce_agg(value, 1, (a, b) -> a * b, (a, b) -> a * b)
      FROM (
          VALUES
              (1, 3),
              (1, 4),
              (1, 5),
              (2, 6),
              (2, 7)
      ) AS t(id, value)
      GROUP BY id;
      -- (1, 60)
      -- (2, 42)

  The state type must be a boolean, integer, floating-point, or date/time/interval.