Postgres - Function to return the intersection of 2 ARRAYs?
SqlPostgresqlSql Problem Overview
In postgresql, you can use the && operator to return t (true) if two arrays have common members, i.e. they overlap. Is there a function/operator that will return what those common members are?
i.e. something like this
select arrray_intersection(ARRAY[1, 4, 2], ARRAY[2, 3]);
ARRAY[2]
Sql Solutions
Solution 1 - Sql
Since 8.4, there are useful builtins in Postgres which make the function from the first answer easier and possibly faster (that's what EXPLAIN tells me, anyway: "(cost=0.00..0.07 rows=1 width=64)" for this query vs. "(cost=0.00..60.02 rows=1 width=64)" for the original one).
The simplified code is:
SELECT ARRAY
(
SELECT UNNEST(a1)
INTERSECT
SELECT UNNEST(a2)
)
FROM (
SELECT array['two', 'four', 'six'] AS a1
, array['four', 'six', 'eight'] AS a2
) q;
and yeah, you can turn it into a function:
CREATE FUNCTION array_intersect(anyarray, anyarray)
RETURNS anyarray
language sql
as $FUNCTION$
SELECT ARRAY(
SELECT UNNEST($1)
INTERSECT
SELECT UNNEST($2)
);
$FUNCTION$;
which you can call as
SELECT array_intersect(array['two', 'four', 'six']
, array['four', 'six', 'eight']);
But you can just as well call it inline too:
SELECT array(select unnest(array['two', 'four', 'six']) intersect
select unnest(array['four', 'six', 'eight']));
Solution 2 - Sql
Try &
instead of &&
See PostgreSQL Docs for more.
Solution 3 - Sql
one another method..
SELECT ARRAY( SELECT * FROM UNNEST( $1 ) WHERE UNNEST = ANY( $2 ) );
Solution 4 - Sql
If you don't mind installing an extension, the intarray extension provides the &
operator to do this as @dwc pointed out.:
SELECT ARRAY[1, 4, 2] & ARRAY[2, 3];
Returns {2}
.
Solution 5 - Sql
You can use this function:
CREATE OR REPLACE FUNCTION intersection(anyarray, anyarray) RETURNS anyarray as $$
SELECT ARRAY(
SELECT $1[i]
FROM generate_series( array_lower($1, 1), array_upper($1, 1) ) i
WHERE ARRAY[$1[i]] && $2
);
$$ language sql;
It should work with any kind of array, and you can use it like this:
SELECT intersection('{4,2,6}'::INT4[], '{2,3,4}'::INT4[]);
Solution 6 - Sql
SELECT ARRAY
(
SELECT a1[s]
FROM generate_series(array_lower(a1, 1), array_upper(a1, 1)) s
INTERSECT
SELECT a2[s]
FROM generate_series(array_lower(a2, 1), array_upper(a2, 1)) s
)
FROM (
SELECT array['two', 'four', 'six'] AS a1, array['four', 'six', 'eight'] AS a2
) q
Works on non-integer arrays too.