我可以在MySQL查询的SELECT部分​​中为带有AS的列添加别名吗？

Question

Ive got a rather complicated query Im running, and before I change a table structure and rewrite some other files I was wondering if anyone could tell me if this is possible.

SELECT
    DISTINCT(users.user_id),
    (resi.curr_state WHERE resi.user_id = "users.user_id") AS current_state
FROM
    ci_users AS users, ci_residents AS resi .....
WHERE stuff HAVING current_state = '".$php_variable_ill_use."'

Im happy to share more code if necessary, but my main question here is can I do that first as statement before the from or even after if it will work.

Answer 1

Something like this might work:

select distinct(users.user_id),
    (select resi.curr_state 
          from ci_residents 
            AS resi_1 where resi_1.user_id = users.user_id) as current_state
FROM
    ci_users AS users, ci_residents AS resi .....
WHERE stuff HAVING current_state = '".$php_variable_ill_use."'

Answer 2

There is a way to conditionally specify the output of a column, yes: you use a CASE statement. But it doesn't look like that's what you're doing. You seem to be specifying a relationship between the resi and users tables, which you could normally accomplish using a JOIN, like this:

SELECT
    DISTINCT(users.user_id),
    resi.curr_state AS current_state
FROM ci_users AS users
JOIN ci_residents AS resi ON users.user_id = resi.user_id

Now, given the logic behind your request, I assume you want all user_ids, but only those states where a state exists. Is that correct? If so, use a LEFT JOIN to include all users records, and include only resi records when an associated record exists.

Please elaborate if you're trying to accomplish something other than what I've illustrated here.