使用联接的SQL查询更新

Question

Here is my SELECT statement that lists what I am trying to change;

SELECT

t1.`qid`,
t1.`gid` as 'incorrect gid',
t1.`question` as 'subquestion',
t1.`parent_qid`,

t2.`qid`,
t2.`gid` as 'correct gid',
t2.`question`

FROM `questions` as t1

LEFT JOIN `questions` as t2 ON t1.`parent_qid` = t2.`qid`

WHERE t1.`sid` = '33844' AND t1.`gid` NOT IN ('1306','1317','1319','1320','1321','1322','1323','1324','1325','1326','1327','1328','1329','1330','1331','1332','1333','1334','1335''1334','1335','1336', '1337','1338','1339','1340')

I want to replace all the 'incorrect gid's with the 'correct gid's using an UPDATE statement.

This is the UPDATE statement I have tried but cannot get to work;

UPDATE `questions` as t1 

SET `gid` = t2.`gid`

FROM

`questions` as t1
LEFT JOIN
`questions` as t2 ON t1.`parent_qid` = t2.`qid`

WHERE t1.`sid` = '33844' AND t1.`gid` NOT IN ('1306','1317','1319','1320','1321','1322','1323','1324','1325','1326','1327','1328','1329','1330','1331','1332','1333','1334','1335''1334','1335','1336', '1337','1338','1339','1340')

Basically, all the rows that are children have an incorrect gid (group id) and need to be fixed. But all rows that refer to the parent (as parent and children are in the same table) will have a correct group id.

So I have to join up the parents groupid, to each child row..

Answer 1

The correct syntax in MySQL:

UPDATE `questions` as t1 LEFT JOIN
       `questions` as t2 ON t1.`parent_qid` = t2.`qid`
    SET t1.`gid` = t2.`gid`
WHERE t1.`sid` = '33844' AND
      t1.`gid` NOT IN ('1306','1317','1319','1320','1321','1322','1323','1324','1325','1326','1327','1328','1329','1330','1331','1332','1333','1334','1335''1334','1335','1336', '1337','1338','1339','1340')