如何在 MySQL 中为 CONCAT 使用别名？

Question

I need to return an alias column eventTitle of which the content can be variable depending on whether another column's value is null . I also need to use the value of that alias to concatenate some other values to return another alias title .

I am running the following query:

$sql = 'SELECT
        CASE WHEN session.title IS NOT NULL
           THEN session.title
           ELSE course.title
        END AS eventTitle,
        CONCAT(eventTitle, "\n", session.city, ", ", session.state) AS title,
        FROM session
        LEFT JOIN course ON course.id = session.course_id';

I get the following error:

Unknown column 'eventTitle' in 'field list'

Answer 1

You can't refer to column aliases in the same SELECT clause. Put it into a subquery. Also, instead of CASE you can use IFNULL (or the more standard COALESCE ).

SELECT eventTitle, CONCAT(eventTitle, "\n", city, ", ", state) AS title
FROM (SELECT IFNULL(session.title, course.title) AS eventTitle, session.city, session.state
      FROM session
      LEFT JOIN course ON course.id = session.course_id) AS subquery

Or you could just use IFNULL twice in the same query.

SELECT IFNULL(session.title, course.title) AS eventTitle,
       CONCAT(IFNULL(session.title, course.title), "\n", session.city, ", ", session.state) AS title
FROM session
LEFT JOIN course ON course.id = session.course_id

BTW, I think you either have the null check or LEFT JOIN backwards. When you use LEFT JOIN , you get NULL in the columns from the second table when there's no match. So the column that will be NULL would be course.title , not session.title .