IF in WHERE mysql query - php

Question

I have this table.
In MySql I want select this user IF key column do not have custom row AND IF key column have custom row -> value column = yes.
This is my query, but it do not work:
where `value` = if(`key` = 'custom', 'yes', '')

Answer 1

you can use the following statement which select any row has key not equal 'custom' or equal 'custom' and value equal 'yes'
Select * from tableName where `key`<> 'custom' or (`key` = 'custom' and `value` = 'yes')

Answer 2

The specification for the query isn't clear, in that it's open to several different interpretations.
I'm thinking the question is about how to return a particular result, not really about using an IF() function in the WHERE clause. (None of the SQL below uses an IF() function in a WHERE clause.)
Here's one possible interpretation of the specification:
By "select this user", we'll take that to mean we want to return a single copy of the user_id column.
By "IF key column do not have custom row", we'll take that to mean that there does not exist such a row for this user_id, then return this user_id. If such a row does exist, then do not return the user.
By "AND IF key column have custom row -> value column = yes.", we'll take that to mean to return this user_id if there is a row for this user_id that matches the condition.
Based on that interpretation, here's an approach to satisfying the specification using conditional aggregation:
SELECT t.user_id
FROM this_table t
GROUP
BY t.user_id
HAVING SUM(1) = SUM(IF(t.key='custom',IF(t.value='yes',1,0),1))
The expression inside the SUM() aggregate gets evaluated for each row. That expression is designed to return either a 1 or a 0, depending on whether the row matches the specified conditions. The SUM() aggregate totals up those 1s and 0s. And then we compare the result to the total number of rows. That tells us if a "matching" row exists or not.
The counts are by user_id, as identified in the GROUP BY clause.
The MySQL IF() function can be replace with a more portable ANSI CASE expression
HAVING SUM(1) = SUM(CASE
WHEN t.key = 'custom'
THEN CASE
WHEN t.value = 'yes'
THEN 1
ELSE 0
END
ELSE 1
END
)
If the intent is to return all of the detail rows for a user, but excluding all user_id that meet the specification, we could use an EXISTS correlated subquery:
SELECT t.user_id
, t.key
, t.value
FROM this_table t
WHERE NOT EXISTS ( SELECT 1
FROM this_table r
WHERE r.user_id = t.user_id
AND r.key = 'custom'
)
OR EXISTS ( SELECT 1
FROM this_table q
WHERE q.user_id = t.user_id
AND q.key = 'custom'
AND q.value = 'yes'
)