如何编辑此 mySQL 查询以在满足条件或未找到行时返回结果？

Question

I have the data table and it is like this:

id ---- user ---- status
1 ----- abc ----- idle
2 ----- def ----- received

that I use the following to get the result

WHERE status ='idle' AND user='abc'

The above is my try to return the result of the query if status = idle' for abc`

However, what I really is to get the result for abc even if it does not exist in the data table and I tried this

 WHERE (status ='idle' or not exists(select * from data where user='2NnG1zvogogH' limit 1)  ) AND user='abc'

without luck.

Is this possible?

Answer 1

I think what you want is:

WHERE (status = 'idle' and user = 'abc')
OR NOT EXISTS (SELECT * FROM data WHERE user = 'abc')

There's no need for LIMIT 1 when a subquery is used with EXISTS , the database knows that it only has to match a single row.

Answer 2

SELECT * FROM (    
    SELECT id, user, status FROM data WHERE status = 'idle'
    UNION
    SELECT * FROM (
        SELECT 0 id, IF((SELECT COUNT(id) FROM data WHERE user = 'abc') = 0, 'abc', NULL) user, 'status' status
    ) tmp
    WHERE user= 'abc'
) tmp2 ORDER BY id

SELECT id, user, status FROM data WHERE status = 'idle' will return all the results in the table that meet this criteria. That one is obvious enough.

SELECT 0 id, IF((SELECT COUNT(id) FROM data WHERE user = 'abc') = 0, 'abc', NULL) user, 'status' status will always return one line. The only difference will be if user = 'abc' or if user = NULL . user will return 'abc' if there is no user = 'abc' in the database.

SELECT * FROM (...) tmp WHERE user= 'abc' just filters the results from the above query so that if the row is user = NULL then it is not included in the rest of the results.

All of this is then ordered in the outermost query.