Error with ORDER BY clause using UNION in MySQL - mysql

Question

I have the following query in MySQL:
(SELECT ue.id, ue.userid, ue.status, ue.timestart, ue.timeend, e.courseid,
e.id AS enrolid, ra.roleid
FROM user_enrolments ue
JOIN enrol e ON e.id = ue.enrolid
JOIN course c ON c.id = e.courseid
JOIN user u ON u.id = ue.userid
JOIN context ct ON ct.instanceid = c.id
LEFT JOIN role_assignments ra ON ra.userid = u.id AND
ra.contextid = ct.id AND
ra.itemid = e.id
WHERE e.customint1 = 1 AND u.deleted = 0 AND
ct.contextlevel = 50 AND (ue.status = 0 OR ue.status = 1))
UNION
(SELECT de.enrolid AS id, de.userid, de.status, de.date_ini, de.date_fin,
de.courseid, de.enrolid, de.roleid
FROM deleted_enrols de
JOIN user u ON u.id = de.userid
WHERE userid = ANY (SELECT userid FROM local_users WHERE clientid = 1))
ORDER BY u.firstname, u.lastname, c.fullname LIMIT 0, 100
If I delete ORBER BY and LIMIT, this query works fine... but the ORDER BY clause gives an error:
Table 'u' from one of the SELECTs cannot be used in global ORDER clause
If I delete the parentheses of both SELECT querys, the error is different:
Table 'u' from one of the SELECTs cannot be used in field list
I have also tried with UNION ALL, but it does not work either.
Any suggestion or clue? Thanks in advance for your time...

Answer 1

The results of your UNION do not include any fields from table 'u', so those results cannot be sorted by table 'u' fields.
You could perhaps perform the UNION and then re-join the results to table 'u', and then use that to sort the results by table 'u' fields. A similar issue exists for sorting on
course.fullname, so that would need to be joined back in, too.
SELECT x.id, x.userid, x.status, x.timestart, x.timeend, x.courseid, x.enrolid, x.roleid
FROM ((SELECT ue.id, ue.userid, ue.status, ue.timestart, ue.timeend, e.courseid,
e.id AS enrolid, ra.roleid
FROM user_enrolments ue
JOIN enrol e ON e.id = ue.enrolid
JOIN course c ON c.id = e.courseid
JOIN user u ON u.id = ue.userid
JOIN context ct ON ct.instanceid = c.id
LEFT JOIN role_assignments ra ON ra.userid = u.id
AND ra.contextid = ct.id
AND ra.itemid = e.id
WHERE e.customint1 = 1 AND u.deleted = 0
AND ct.contextlevel = 50 AND (ue.status = 0 OR ue.status = 1))
UNION
(SELECT de.enrolid AS id, de.userid, de.status, de.date_ini, de.date_fin,
de.courseid, de.enrolid, de.roleid
FROM deleted_enrols de
JOIN user u ON u.id = de.userid
WHERE userid = ANY (SELECT userid FROM local_users WHERE clientid = 1))
) x
JOIN user z ON z.id = x.userid
JOIN course d ON d.id = x.courseid
ORDER BY z.firstname, z.lastname, d.fullname LIMIT 0, 100

Answer 2

Assuming you want to sort the whole lot, try parentheses round the whole query with the ORDER BY done afterwards:
select id, userid, status, timestart, timeend, courseid, enrolid, roleid from
((SELECT ue.id, ue.userid, ue.status, ue.timestart, ue.timeend, e.courseid,
e.id AS enrolid, ra.roleid, u.firstname, u.lastname, c.fullname
FROM user_enrolments ue
JOIN enrol e ON e.id = ue.enrolid
JOIN course c ON c.id = e.courseid
JOIN user u ON u.id = ue.userid
JOIN context ct ON ct.instanceid = c.id
LEFT JOIN role_assignments ra ON ra.userid = u.id AND
ra.contextid = ct.id AND
ra.itemid = e.id
WHERE e.customint1 = 1 AND u.deleted = 0 AND
ct.contextlevel = 50 AND (ue.status = 0 OR ue.status = 1))
UNION
(SELECT de.enrolid AS id, de.userid, de.status, de.date_ini, de.date_fin,
de.courseid, de.enrolid, de.roleid, u.firstname, u.lastname, ' ' as fullname
FROM deleted_enrols de
JOIN user u ON u.id = de.userid
WHERE userid = ANY (SELECT userid FROM local_users WHERE clientid = 1))) s1
ORDER BY firstname, lastname, fullname LIMIT 0, 100
(obviously fullname in the second SELECT statement would be populated however seems sensible)

Answer 3

You need to include the data to be ordered by in the selects of the unioned queries; an ORDER BY following a UNION is handled as if it were SELECT * FROM (unions) ORDER BY ... so anything not coming out of the union cannot be used for ordering.
Ironically, a query similar to that is the key to getting what you want though, with something like
SELECT x, y, z
FROM (
SELECT x, y, z, somethingIdontactuallywant
FROM blah
UNION
SELECT a, b, c, somethingIdontactuallywant
FROM blah2
) AS u
ORDER BY u.somethingIdontactuallywant

Answer 4

As mysql documentation on union says:
This kind of ORDER BY cannot use column references that include a
table name (that is, names in tbl_name.col_name format). Instead,
provide a column alias in the first SELECT statement and refer to the
alias in the ORDER BY. (Alternatively, refer to the column in the
ORDER BY using its column position. However, use of column positions
is deprecated.)
Also, if a column to be sorted is aliased, the ORDER BY clause must
refer to the alias, not the column name.
So, do not refer to any table names and use columns that are actually in the resultset of the union.