Question

I have this problem:

I have two tables: one with user data and one with postal codes assigned to user (one user can have multiple postal codes).

Table with postal codes has two columns – start and end. First I need to select START and END and separate them eg. with -. I know I can use CONCAT_WS to do this, but I also need to take this concatenated strings (there are multiple rows) and concatenate them into one row.

I already have this query:

SELECT pc.user_id, CONCAT_WS('-', pc.start, pc.end) FROM postal_codes pc, users u WHERE u.id=pc.user_id

But s you expect it gives me results like this: (sorry I don’t know how to insert tables here)

row | user_id | postal range
============================
1   | 1       | AAAA-BBBB
----------------------------
2   | 1       | CCCC-DDDD
----------------------------
3   | 1       | MMMM-NNNN
----------------------------
4   | 2       | CCCC-DDDD
----------------------------
5   | 2       | EEEE-FFFF
----------------------------

and I need it to be like this:

row | user_ID | postal_range
----------------------------
1   | 1       | AAAA-BBBB, CCCC-DDDD, MMMM-NNNN
----------------------------
2   | 2       | CCCC-DDDD, EEEE-FFFF
----------------------------

I know I can use GROUP_CONCAT if i want to concatenate multiple rows – but if I wrap result of previous query into GROUP_CONCAT like this:

SELECT pc.user_id, (GROUP_CONCAT((SELECT CONCAT_WS('-', pc.start, pc.end) FROM bs_postal_codes pc, bs_users u WHERE u.id=pc.user_id) SEPARATOR ', ')) FROM bs_postal_codes pc

It gives me error: Subquery returns more than 1 row

It would really help me if someone give me a hand with this, thanks.

Answer 1

GROUP_CONCAT, as all aggregate functions, work within the query they are part of (the outer query). You cannot add a SELECT statement as a parameter that returns more than one row.

try this:

SELECT 
  pc.user_id, 
  GROUP_CONCAT(CONCAT_WS('-', pc.start, pc.end) SEPARATOR ', ') as codes
FROM 
  bs_postal_codes pc
GROUP BY
  pc.user_id

Addition (see comments):

1.

SELECT 
  u.id, 
  /* Optional other user fields. Add them to Group By too, */
  GROUP_CONCAT(CONCAT_WS('-', pc.start, pc.end) SEPARATOR ', ') as codes
FROM 
  users u
  LEFT JOIN bs_postal_codes pc ON pc.user_id = u.id
GROUP BY
  u.id

2.

SELECT 
  u.id, 
  /* Optional other user fields. Add them to Group By too, */
  (SELECT
    GROUP_CONCAT(CONCAT_WS('-', pc.start, pc.end) SEPARATOR ', ')
  FROM 
    bs_postal_codes pc
  WHERE
    pc.user_id = u.id) as codes
FROM 
  users u