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.
Ordering during aggregation
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)
Filtering during aggregation
The FILTER keyword can be used to remove rows from aggregation processing
with a condition expressed using a WHERE clause. This is evaluated for each
row before it is used in the aggregation and is supported for all aggregate
functions.
aggregate_function(...) FILTER (WHERE <condition>)
A common and very useful example is to use FILTER to remove nulls from
consideration when using array_agg
SELECT array_agg(name) FILTER (WHERE name IS NOT NULL)
FROM region;
As another example, imagine you want to add a condition on the count for Iris flowers, modifying the following query:
SELECT species,
count(*) AS count
FROM iris
GROUP BY species;
species | count
-----------+-------
setosa | 50
virginica | 50
versicolor | 50
If you just use a normal WHERE statement you lose information:
SELECT species,
count(*) AS count
FROM iris
WHERE petal_length_cm > 4
GROUP BY species;
species | count
-----------+-------
virginica | 50
versicolor | 34
Using a filter you retain all information:
SELECT species,
count(*) FILTER (where petal_length_cm > 4) AS count
FROM iris
GROUP BY species;
species | count
-----------+-------
virginica | 50
setosa | 0
versicolor | 34
General aggregate functions
any_value
any_value(x) -> [same as input]
Returns an arbitrary non-null value x, if one exists. x can be any
valid expression. This allows you to return values from columns that are not
directly part of the aggregation, inluding expressions using these columns,
in a query.
For example, the following query returns the customer name from the name
column, and returns the sum of all total prices as customer spend. The
aggregation however uses the rows grouped by the customer identifier
custkey a required, since only that column is guaranteed to be unique:
SELECT sum(o.totalprice) as spend,
any_value(c.name)
FROM tpch.tiny.orders o
JOIN tpch.tiny.customer c
ON o.custkey = c.custkey
GROUP BY c.custkey;
ORDER BY spend;
arbitrary
arbitrary(x) -> [same as input]
Returns an arbitrary non-null value of x, if one exists. Identical to any_value.
array_agg
array_agg(x) -> array<[same as input]>
Returns an array created from the input x elements.
avg
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
bool_and(boolean) -> boolean
Returns TRUE if every input value is TRUE, otherwise FALSE.
bool_or
bool_or(boolean) -> boolean
Returns TRUE if any input value is TRUE, otherwise FALSE.
checksum
checksum(x) -> varbinary
Returns an order-insensitive checksum of the given values.
count
count(*) -> bigint
Returns the number of input rows.
count(x) -> bigint
Returns the number of non-null input values.
count_if
count_if(x) -> bigint
Returns the number of TRUE input values.
This function is equivalent to count(CASE WHEN x THEN 1 END).
every
every(boolean) -> boolean
This is an alias for bool_and.
geometric_mean
geometric_mean(x) -> double
Returns the geometric mean of all input values.
listagg
listagg(x, separator) -> varchar
Returns the concatenated input values, separated by the separator string.
Synopsis:
LISTAGG( expression [, separator] [ON OVERFLOW overflow_behaviour])
WITHIN GROUP (ORDER BY sort_item, [...]) [FILTER (WHERE condition)]
If separator is not specified, the empty string will be used as separator.
In its simplest form the function looks like:
SELECT listagg(value, ',') WITHIN GROUP (ORDER BY value) csv_value
FROM (VALUES 'a', 'c', 'b') t(value);
and results in:
csv_value
-----------
'a,b,c'
The overflow behaviour is by default to throw an error in case that the length of the output
of the function exceeds 1048576 bytes:
SELECT listagg(value, ',' ON OVERFLOW ERROR) WITHIN GROUP (ORDER BY value) csv_value
FROM (VALUES 'a', 'b', 'c') t(value);
There exists also the possibility to truncate the output WITH COUNT or WITHOUT COUNT
of omitted non-null values in case that the length of the output of the
function exceeds 1048576 bytes:
SELECT listagg(value, ',' ON OVERFLOW TRUNCATE '.....' WITH COUNT) WITHIN GROUP (ORDER BY value)
FROM (VALUES 'a', 'b', 'c') t(value);
If not specified, the truncation filler string is by default '...'.
This aggregation function can be also used in a scenario involving grouping:
SELECT id, listagg(value, ',') WITHIN GROUP (ORDER BY o) csv_value
FROM (VALUES
(100, 1, 'a'),
(200, 3, 'c'),
(200, 2, 'b')
) t(id, o, value)
GROUP BY id
ORDER BY id;
results in:
id | csv_value
-----+-----------
100 | a
200 | b,c
This aggregation function supports filtering during aggregation for scenarios where the aggregation for the data not matching the filter condition still needs to show up in the output:
SELECT
country,
listagg(city, ',')
WITHIN GROUP (ORDER BY population DESC)
FILTER (WHERE population >= 10_000_000) megacities
FROM (VALUES
('India', 'Bangalore', 13_700_000),
('India', 'Chennai', 12_200_000),
('India', 'Ranchi', 1_547_000),
('Austria', 'Vienna', 1_897_000),
('Poland', 'Warsaw', 1_765_000)
) t(country, city, population)
GROUP BY country
ORDER BY country;
results in:
country | megacities
---------+-------------------
Austria | NULL
India | Bangalore,Chennai
Poland | NULL
The current implementation of listagg function does not support window frames.
max
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.
max_by
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
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.
min_by
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.
sum
sum(x) -> [same as input]
Returns the sum of all input values.
Bitwise aggregate functions
bitwise_and_agg
bitwise_and_agg(x) -> bigint
Returns the bitwise AND of all input values in 2’s complement representation.
bitwise_or_agg
bitwise_or_agg(x) -> bigint
Returns the bitwise OR of all input values in 2's complement representation.
Map aggregate functions
histogram
histogram(x) -> map<K,bigint>
Returns a map containing the count of the number of times each input value occurs.
map_agg
map_agg(key, value) -> map<K,V>
Returns a map created from the input key / value pairs.
map_union
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.
For example, take the following histogram function that creates multiple maps from the Iris dataset:
SELECT histogram(floor(petal_length_cm)) petal_data
FROM memory.default.iris
GROUP BY species;
petal_data
-- {4.0=6, 5.0=33, 6.0=11}
-- {4.0=37, 5.0=2, 3.0=11}
-- {1.0=50}
You can combine these maps using map_union:
SELECT map_union(petal_data) petal_data_union
FROM (
SELECT histogram(floor(petal_length_cm)) petal_data
FROM memory.default.iris
GROUP BY species
);
petal_data_union
--{4.0=6, 5.0=2, 6.0=11, 1.0=50, 3.0=11}
multimap_agg
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
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_most_frequent
approx_most_frequent(buckets, value, capacity) -> map<[same as value], bigint>
Computes the top frequent values up to buckets elements approximately.
Approximate estimation of the function enables us to pick up the frequent
values with less memory. Larger capacity improves the accuracy of
underlying algorithm with sacrificing the memory capacity. The returned
value is a map containing the top elements with corresponding estimated
frequency.
The error of the function depends on the permutation of the values and its cardinality. We can set the capacity same as the cardinality of the underlying data to achieve the least error.
buckets and capacity must be bigint. value can be numeric
or string type.
The function uses the stream summary data structure proposed in the paper Efficient Computation of Frequent and Top-k Elements in Data Streams by A. Metwalley, D. Agrawl and A. Abbadi.
approx_percentile
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 percentage. Weights must be
greater or equal to 1. Integer-value weights can be thought of as a replication
count for the value x in the percentile set. The value of percentage must be
between zero and one and 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. Weights must be greater or equal to 1. Integer-value weights can
be thought of as a replication count for the value x in the percentile
set. Each element of the percentages array must be between zero and one, and the array
must be constant for all input rows.
approx_set
approx_set(x) -> HyperLogLog
merge
merge(x) -> HyperLogLog
merge(qdigest(T)) -> qdigest(T)
See Quantile digest functions.
merge(tdigest) -> tdigest
numeric_histogram
numeric_histogram(buckets, value) -> map<double, double>
Computes an approximate histogram with up to buckets number of buckets
for all values. This function is equivalent to the variant of
numeric_histogram that takes a weight, with a per-item weight of 1.
numeric_histogram(buckets, value, weight) -> map<double, double>
Computes an approximate histogram with up to buckets number of buckets
for all values 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.
qdigest_agg
qdigest_agg(x) -> qdigest([same as x])
qdigest_agg(x, w) -> qdigest([same as x])
qdigest_agg(x, w, accuracy) -> qdigest([same as x])
tdigest_agg
tdigest_agg(x) -> tdigest
tdigest_agg(x, w) -> tdigest
Statistical aggregate functions
corr
corr(y, x) -> double
Returns correlation coefficient of input values.
covar_pop
covar_pop(y, x) -> double
Returns the population covariance of input values.
covar_samp
covar_samp(y, x) -> double
Returns the sample covariance of input values.
kurtosis
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
regr_intercept(y, x) -> double
Returns linear regression intercept of input values. y is the dependent
value. x is the independent value.
regr_slope
regr_slope(y, x) -> double
Returns linear regression slope of input values. y is the dependent
value. x is the independent value.
skewness
skewness(x) -> double
Returns the Fisher’s moment coefficient of skewness of all input values.
stddev
stddev(x) -> double
This is an alias for stddev_samp.
stddev_pop
stddev_pop(x) -> double
Returns the population standard deviation of all input values.
stddev_samp
stddev_samp(x) -> double
Returns the sample standard deviation of all input values.
variance
variance(x) -> double
This is an alias for var_samp.
var_pop
var_pop(x) -> double
Returns the population variance of all input values.
var_samp
var_samp(x) -> double
Returns the sample variance of all input values.
Lambda aggregate functions
reduce_agg
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, char, varchar or date/time/interval.