SELECT AS别名上的LEFT JOIN导致在WHERE语句上找不到列

Question

I want to search user by 'user_location' which is an alias of location.detail And I made this following query :

SELECT user.*, location.detail as user_location 
FROM user 
LEFT JOIN location
ON user.id = location.user_id
WHERE user_location = 'jakarta';

It's resulting error says,

Column not found: 1054 Unknown column 'user_location' in 'where clause' 

Why is it? what is the solution? Any answer will be highly appreciated!

Answer 1

You can't user an alias in the where in the same query you created it. So you should do:

SELECT user.*, location.detail as user_location 
  FROM user 
        LEFT JOIN location
          ON user.id = location.user_id
 WHERE location.detail = 'jakarta';

If you still want to use that column you have to wrap it in a subquery:

select * from
(
SELECT user.*, location.detail as user_location 
  FROM user 
        LEFT JOIN location
          ON user.id = location.user_id
) a
where user_location = 'jakarta';

As @spencer7593 mentioned you can use an alias in the same query in the HAVING clause.

Answer 2

The obvious and correct solution is to repeat the expression in the WHERE clause. Sometimes, the expressions are more complicated (think distance calculations). MySQL has an extension to SQL that allows you to use HAVING for this purpose:

SELECT u.*, l.detail as user_location 
FROM user u LEFT JOIN
     location l
     ON u.id = l.user_id
HAVING user_location = 'jakarta';

I don't actually recommend it for this particular query (after all, "user_location" is longer to type out than "l.detail"). Under some circumstances, though, this can be convenient.

More important than these considerations is that your query is really an INNER JOIN . You should specify it as such:

SELECT u.*, l.detail as user_location 
FROM user u INNER JOIN
     location l
     ON u.id = l.user_id
WHERE l.detail = 'jakarta';

The filtering condition filters out all unmatched rows. Using a LEFT JOIN inappropriately confuses people and might confuse the optimizer.

Answer 3

You cannot use aliases in Where clause in mysql. Try

SELECT user.*, location.detail as user_location 
FROM user 
LEFT JOIN location
ON user.id = location.user_id
WHERE location.detail = 'jakarta';