MySQL union and order by help

问题

The below code works but when i change Order by id to Order by s.id, i get this error

Unknown column 's.id' in 'order clause'

$construct =  "SELECT child.* FROM products child LEFT JOIN products parent on parent.name=child.parent INNER JOIN subscribe s ON (s.productid = parent.id) WHERE s.username='$logged' AND s.type='0'
        UNION
        SELECT child.* FROM products child LEFT JOIN products parent on parent.sid=child.sid INNER JOIN subscribe s ON (s.productid = parent.id) WHERE s.username='$logged' AND parent.keyword != child.name AND s.type='1'
        ORDER BY s.id DESC";

How can i change the code so it is ordered by s.id, which is the subscribe table's id?

回答1:

MySQL is trying to apply the ORDER BY to the UNION but the UNION only has the child columns (without the child. prefix at that), there is no s.id in the UNION. But you can add one:

SELECT child.*, s.id as sid ...
UNION
SELECT child.*, s.id as sid ...
ORDER BY sid DESC

You need to give it an alias as the UNION will strip off the table name or alias prefix. If there is an sid column in child then use something else as the alias for s.id.

回答2:

"INNER JOIN subscribe s"

It looks like maybe you want:

INNER JOIN `subscribe` AS `s`

if that doesn't work, please post the output of:

DESCRIBE subscribe;

来源：https://stackoverflow.com/questions/7278959/mysql-union-and-order-by-help