Postgres JOIN with unnest

问题

Assume I have following tables:

table: followers_arrays

   id   |  array
--------+---------
    1   | {3,4,5}


table: small_profiles

   id   | username |  pic    
--------+----------+-------
    3   |   aaaa   | abcd
    4   |   bbbb   | abcd
    5   |   cccc   | abcd

I would like to print followers_array with populated data from small_profiles using simple JOINs.

At first, I'm using unnest function like this:

SELECT id, unnest(followers_array) AS elem FROM followers_arrays

And it gives me about right result:

   id   |  elem  
--------+--------
    1   |    3
    1   |    4
    1   |    5

Now, from my understanding I just need to join this data to small_profiles ON small_profiles.id key like this:

SELECT id, unnest(followers_array) AS elem 
FROM followers_arrays 
JOIN small_profiles ON small_profiles.instagram_id = elem

However it seems that during JOIN, column elem is not created yet because I get following error: ERROR: column "elem" does not exist

Any thoughts how should I rearrange my query? Thanks

回答1:

That is bad design but here is your answer:

select f.id, f.follower, s.username, s.pic
from
    (
        select id, unnest("array") as follower
        from followers_arrays
    ) f
    inner join
    small_profiles s on f.follower = s.id

回答2:

I prefer to use Common Table Expressions over Subqueries:

WITH unnested_arr_1 AS (
    SELECT unnest(ARRAY[1, 2, 3]) array_1_item
)
,unnested_arr_2 AS (
    SELECT unnest(ARRAY[2, 3, 4]) array_2_item
)
SELECT  *
FROM    unnested_arr_1 arr1
    FULL OUTER JOIN unnested_arr_2 arr2 ON arr1.array_1_item=arr2.array_2_item

produces:

array_1_item |array_2_item |
-------------|-------------|
1            |[NULL]       |
2            |2            |
3            |3            |
[NULL]       |4            |

If joining only unnested arrays, then the above query can be simplified as follows:

SELECT * 
FROM   unnest(
    ARRAY[1, 2, 3]
   ,ARRAY[2, 3, 4]
) as U(array_1_item , array_2_item );

来源：https://stackoverflow.com/questions/33466823/postgres-join-with-unnest

标签

database

postgresql

join

relation

unnest