PostgreSQL: get count of occurrences of specified element in array - sql

Question

I need to calculate the count of occurrences of specified element in array, something like:
elem_occurrences_count(ARRAY[a,b,c,a,a], a) = 3
elem_occurrences_count(ARRAY[a,b,c], d) = 0
Is there any function in PostgreSQL that can be used to solve the problem? Any help is appreciated.

Answer 1

You will need to unnest the array and then count the occurrences.
with elements (element) as (
select unnest(ARRAY['a','b','c','a','a'])
)
select count(*)
from elements
where element = 'a';
This can easily be embedded into a function:
create or replace function count_elements(elements text[], to_find text)
returns bigint
as
$body$
select count(*)
from unnest(elements) element
where element = to_find;
$body$
language sql;
Update
Since Postgres 9.5 this can also be done using array_positions() which returns an array of positions where an element was found. The length of that array is the number of occurrences:
select cardinality(array_positions(ARRAY['a','b','c','a','a'], 'a'));

Answer 2

9.5+
There is an easier method now
SELECT
sArray,
c,
coalesce(array_length( array_positions(sArray, c), 1 ),0) AS count
FROM ( VALUES
(ARRAY['a','b','c','a','a'], 'a'),
(ARRAY['a','b','c'], 'd')
) AS t(sArray,c);
sarray | c | count
-------------+---+-------
{a,b,c,a,a} | a | 3
{a,b,c} | d | 0
(2 rows)

Answer 3

The occurrence of all elements in an array can be found with this query:
SELECT count(id), UNNEST(array) as element
FROM myTable
GROUP BY element;
To count the occurrence of a specific element, for example 'c', add a WHERE clause:
SELECT count(id), UNNEST(array) as element
FROM myTable
WHERE EXISTS (SELECT * FROM UNNEST(array) AS x WHERE x='c')
GROUP BY element;

Answer 4

You can use the FILTER clause to count the occurrences.
Let's say we have a questions table with tags (array type) and you want to count the questions with postgresql tag:
SELECT COUNT(*) FILTER (WHERE '{postgresq}' <# (tags)) as tagCount
FROM posts;

Answer 5

More generic function is here;
CREATE FUNCTION count_array_elements (
i_elements pg_catalog.anyarray,
i_element_to_find pg_catalog.anyelement,
out count bigint
)
RETURNS bigint AS
$body$
BEGIN
SELECT count(*) INTO count
FROM unnest(i_elements) u
WHERE u = i_element_to_find;
END;
$body$
LANGUAGE 'plpgsql'
IMMUTABLE
RETURNS NULL ON NULL INPUT;
With this way, we can query like this one below;
SELECT * FROM count_array_elements(array [ TRUE, TRUE, FALSE, FALSE, FALSE ], TRUE);

Answer 6

Thanks to all contributors here, I learnt a few things.
I am building on work of others in this thread and others in stackoverflow.
I tried to create a function that will count for all the unique elements in the array.
I was targeting returning a json but it seems you can only return as SETOF.
result of count_element_3
CREATE OR REPLACE FUNCTION count_element_3(str_array text[])
RETURNS setof text
AS
$$
DECLARE
unique_element_array text[];
cardinality_array int[];
retArray text[];
BEGIN
-- Find unique items first
unique_element_array := array(select distinct unnest(str_array));
FOR I IN array_lower(unique_element_array, 1)..array_upper(unique_element_array, 1)
LOOP
cardinality_array[I] := (select cardinality(array_positions(str_array, unique_element_array[I])));
retArray[I] := concat(unique_element_array[I],':',cardinality_array[I]);
END LOOP;
RETURN QUERY SELECT(retArray::text);
END;
$$
LANGUAGE plpgsql
VOLATILE
RETURNS NULL ON NULL INPUT;
with t1 as (SELECT
sArray,
c,
coalesce(array_length( array_positions(sArray, c), 1 ),0) AS count
FROM ( VALUES
(ARRAY['a','b','c','a','a'], 'a'),
(ARRAY['a','b','c'], 'd')
) AS t(sArray,c)
)
select sarray, count_element_3(sarray) from t1
sarray count_element_3
text[] text
-------------------------------------
"{a,b,c,a,a}" "{c:1,a:3,b:1}"
"{a,b,c}" "{c:1,a:1,b:1}"