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'
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
.
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