WHILE conditions are not working properly in MYSQL query? - mysql

Question

I am learning MySQL through self-practice. In a project, I want to create a transfer module using MySQL (phpMyAdmin). Unfortunately, the WHERE conditions are not working well. I execute the query using the XAMPP application. A part of the query is -
SELECT
*
FROM
(SELECT
`emp_id`,
`emp_name`,
`present_posting`,
`curr_zone`,
`office_ID1` AS `new_office`,
`Zone1` AS `new_zone`,
`office_ID1` AS `C1`,
`post`,
`preference`,
`curr_zone_id`
FROM
`transfer_applications`
WHERE
`Mutual Accepted` != 'Mutual Accepted'
ORDER BY `apID` ASC) `aa`
LEFT JOIN
(SELECT
`zone_ID`, `ZONE`, `office_ID`, `office_Vacancy`
FROM
`vacancy`) `ab` ON `aa`.`C1` = `ab`.`office_ID`
LEFT JOIN
(SELECT
`zid`, `max_min_vacancy`
FROM
`capping_vacancy`) `ac` ON `aa`.`new_zone` = `ac`.`zid`
WHERE
((`curr_zone_id` != `new_zone`
AND (`max_min_vacancy` < 150
AND `max_min_vacancy` > 1)
AND `station_Vacancy` > 0
AND `post` = 4
AND `apID` = x + 1)
OR (`curr_zone_id` = `new_zone`
AND `station_Vacancy` > 0))
The problem is that it allows transfer even if there is no vacancy available that is the minimum capping in WHERE ( max_min_vacancy > 1 ) is not working. I am unable to find out the reason why it skips this condition while all other conditions in the WHERE are working fine. Kindly help me to find out the mistake. Thanks.

Answer 1

The WHERE clause is connected by an OR, so if the first Boolean expression is FALSE (ie. max_min_vacancy <=1), it is still TRUE as long as the second boolean expression returns TRUE.
...
WHERE
(
(
`curr_zone_id` != `new_zone` AND
(
`max_min_vacancy` < 150 AND
`max_min_vacancy` > 1 -- if max_min_vacancy <= 1 --> FALSE
) AND
`station_Vacancy` > 0 AND
`post` = 4 AND
`apID` = x + 1
)
OR ( -- But this condition is TRUE
`curr_zone_id` = `new_zone` AND
`station_Vacancy` > 0
)
)