Postgres - Function to return the intersection of 2 ARRAYs?

Question

In postgresql, you can use the && operator to return t (true) if two arrays have common members, ie they overlap. Is there a function/operator that will return what those common members are?

ie something like this

select arrray_intersection(ARRAY[1, 4, 2], ARRAY[2, 3]);
ARRAY[2]

Answer 1

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']));

Answer 2

Try & instead of &&

See PostgreSQL Docs for more.

Answer 3

另一种方法..

SELECT ARRAY( SELECT * FROM UNNEST( $1 ) WHERE UNNEST = ANY( $2 ) );

Answer 4

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} .

Answer 5

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.

Answer 6

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[]);

Answer 7

aother option, for those using for example postgresql, is to take benefits of the jsonb_path_exists tool to check for example if at least one of the elements of an array is also available in another array!

assuming for example here below that we have a table activity_role containing a column "rights" format JSONB, in which we could find for example

table: activity_role

id	rights
1	{"folders":["write","read","assign"]}

if we want to check if for example id 1 has at least one of the following rights ['sign', approve', 'write'], we simply run

SELECT jsonb_path_exists(ar.rights, '$.folders[*] ? (@ == $requested_right)',jsonb_build_object('requested_right', array['sign', 'approve', 'write']) AS authorized FROM activity_rights ar WHERE ar.id = 1;

result will be

authorized

---

true

because at least 'write' matches in the array1 (rights->'folders') and in the array2 ( array['sign', approve', 'write'] )

Answer 8

How the inline operation && would look like in SELECT statement where we are selecting from a 2 tables and filtering only where two array columns intersect (at least 1 value)

SELECT
    table_1 .list::text[],
    table_2 .list::text[]
FROM
    table_1,
    table_2
WHERE
    table_1.list::text[] && table_2.list::text[]