MySql: #1221 - Incorrect usage of UNION and ORDER BY

Question

I have the following query which is intended to retrieve the newest 3 comments for each post in an array of posts.

SELECT * FROM comment AS tbl WHERE sscm_id IN 
(
 SELECT sscm_id FROM comment AS t1 WHERE t1.sscm_oid = 4 
 ORDER BY t1.sscm_id DESC LIMIT 3
 UNION ALL
 SELECT sscm_id FROM comment AS t2 WHERE t2.sscm_oid = 3 
 ORDER BY t2.sscm_id DESC LIMIT 3
)

which gives me the following error:

1221 - Incorrect usage of UNION and ORDER BY

Some searching & googling seemed to suggest that placing () around each subquery might solve this problem but changing the query to

SELECT * FROM comment AS tbl WHERE sscm_id IN
(
 (SELECT sscm_id FROM comment AS t1 WHERE t1.sscm_oid = 4 
 ORDER BY t1.sscm_id DESC LIMIT 3)
 UNION ALL
 (SELECT sscm_id FROM comment AS t2 WHERE t2.sscm_oid = 3 
 ORDER BY t2.sscm_id DESC LIMIT 3)
)

but this gives me the following error:

1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'UNION ALL (SELECT sscm_id FROM bpsocial_comment AS t2 WHERE t2.sscm_oid = 3 OR' at line 4

So: how can I get the UNION in combination with the ORDER BY in the subqueries to work?

Thanks R

• ypercubeᵀᴹ: Is sscm_id the primary key of the table?

Answer 1

There are several ways to pass the IN / LIMIT limitation and the UNION / ORDER BY syntax requirements.

Wrap the Union in a derived table:

SELECT * FROM comment AS tbl WHERE sscm_id IN
 ( SELECT sscm_id
 FROM 
 ( ( SELECT sscm_id FROM comment AS t1 WHERE t1.sscm_oid = 4 
 ORDER BY t1.sscm_id DESC LIMIT 3)
 UNION ALL
 ( SELECT sscm_id FROM comment AS t2 WHERE t2.sscm_oid = 3 
 ORDER BY t2.sscm_id DESC LIMIT 3)
 ) AS tmp 
 ) ;

Use JOIN instead of WHERE / IN:
UNION is required here instead of UNION ALL to avoid duplicates

SELECT tbl.* 
FROM comment AS tbl 
 JOIN 
 ( ( SELECT sscm_id FROM comment AS t1 WHERE t1.sscm_oid = 4 
 ORDER BY t1.sscm_id DESC LIMIT 3)
 UNION 
 ( SELECT sscm_id FROM comment AS t2 WHERE t2.sscm_oid = 3 
 ORDER BY t2.sscm_id DESC LIMIT 3)
 ) AS tmp 
 ON tmp.sscm_id = tbl.sscm_id ;