Where clause using alias column with IF and IFNULL - mysql

Question

I have an mysql query like below:
SELECT
sppt_ticket.*,
IF(sppt_read_support_ticket.ID_aks_user IS NULL,'N', 'Y') AS `read_status`,
IFNULL(readcomment.total_comment, 0) AS unread_comment
FROM
sppt_ticket
LEFT JOIN
sppt_read_support_ticket ON
sppt_ticket.ID_support_ticket = sppt_read_support_ticket.ID_support_ticket AND
ID_aks_user = 1
LEFT JOIN
(SELECT
sppt_comment.ID_support_ticket, SUM(IF(sppt_read_comment.ID_aks_user IS NULL, 1, 0))
AS
total_comment
FROM
sppt_comment
LEFT JOIN
sppt_read_comment
ON
sppt_comment.ID_comment = sppt_read_comment.ID_comment
AND
sppt_read_comment.ID_aks_user = 1
GROUP BY
sppt_comment.ID_support_ticket) AS readcomment ON readcomment.ID_support_ticket = sppt_ticket.ID_support_ticket
What I want to get in where clause is like this
WHERE read_status = 'Y'
I've tried using subquery, but still I didn't get it..
any help?

Answer 1

Have you tried this:
SELECT * FROM
(
-- your original query as a table
SELECT
sppt_ticket.*,
IF(sppt_read_support_ticket.ID_aks_user IS NULL,'N', 'Y') AS `read_status`,
IFNULL(readcomment.total_comment, 0) AS unread_comment
FROM
sppt_ticket
LEFT JOIN
sppt_read_support_ticket ON
sppt_ticket.ID_support_ticket = sppt_read_support_ticket.ID_support_ticket AND
ID_aks_user = 1
LEFT JOIN
(SELECT
sppt_comment.ID_support_ticket, SUM(IF(sppt_read_comment.ID_aks_user IS NULL, 1, 0))
AS
total_comment
FROM
sppt_comment
LEFT JOIN
sppt_read_comment
ON
sppt_comment.ID_comment = sppt_read_comment.ID_comment
AND
sppt_read_comment.ID_aks_user = 1
GROUP BY
sppt_comment.ID_support_ticket) AS readcomment ON readcomment.ID_support_ticket = sppt_ticket.ID_support_ticket
)
as temptable
where read_status = 'Y' -- this should work
Or you can use HAVING instead of WHERE if you do not want to treat your query as a table:
SELECT
sppt_ticket.*,
IF(sppt_read_support_ticket.ID_aks_user IS NULL,'N', 'Y') AS `read_status`,
IFNULL(readcomment.total_comment, 0) AS unread_comment
FROM
sppt_ticket
LEFT JOIN
sppt_read_support_ticket ON
sppt_ticket.ID_support_ticket = sppt_read_support_ticket.ID_support_ticket AND
ID_aks_user = 1
LEFT JOIN
(SELECT
sppt_comment.ID_support_ticket, SUM(IF(sppt_read_comment.ID_aks_user IS NULL, 1, 0))
AS
total_comment
FROM
sppt_comment
LEFT JOIN
sppt_read_comment
ON
sppt_comment.ID_comment = sppt_read_comment.ID_comment
AND
sppt_read_comment.ID_aks_user = 1
GROUP BY
sppt_comment.ID_support_ticket) AS readcomment ON readcomment.ID_support_ticket = sppt_ticket.ID_support_ticket
HAVING read_status = 'Y' -- use HAVING instead of WHERE
The reason why treating your original query as a table works is because of the way values are evaluated in the query. In your original query, the alias read_status cannot be used with the WHERE clause because the actual value might not yet be known when the WHERE clause is evaluated. As documented in Section B.1.5.4, “Problems with Column Aliases”. Treating it as a table ensures that the value for read_status has already been evaluated.
For the HAVING approach, MySQL created an extension to standard SQL that permits references in the HAVING clause to aliased expressions in the select list.