Column not found in Select statement inside select statement - mysql

Question

Trying to select one row at a time from this query (for example where the rank = 1). However, it doesn't work because "unknown column 'rank'. if i change it to "WHERE id = 1" then it works, but for some reason it doesn't know what rank is even tho it is set
SET #rownum=0;
SELECT #rownum := #rownum + 1 AS rank, id, client, date, time, pickupCity, pickupState
FROM (
SELECT r.id, CONCAT(c.fname, ' ', c.lname) as client, r.date,
LOWER(TIME_FORMAT(r.time, '%l:%i%p')) as time, r.pickupCity, r.pickupState
FROM request r
INNER JOIN client c ON r.client = c.id
INNER JOIN pickup p ON r.id = p.request
INNER JOIN driver d ON d.id = p.driver
WHERE date = '2018-04-18' AND d.id = 1
GROUP BY time
) AS tab
HAVING rank = 1;

Answer 1

In MySQL, you can do this using HAVING:
SELECT #rownum := #rownum + 1 AS rank, id, client, date, time, pickupCity, pickupState
FROM (SELECT r.id, CONCAT(c.fname, ' ', c.lname) as client, r.date,
LOWER(TIME_FORMAT(r.time, '%l:%i%p')) as time, r.pickupCity,
r.pickupState
FROM request r JOIN
client c
ON r.client = c.id JOIN
driver d
ON ?
pickup p
ON d.id = p.driver
WHERE date = '2018-04-18' AND d.id = 1
GROUP BY time
) t CROSS JOIN
(SELECT #rank := 0) params
HAVING rank = 1;
Notes:
The ?? is for the missing JOIN conditions.
I went through the effort of fixing your JOIN syntax. You should go through the effort of learning proper, explicit, standard JOIN syntax.
You can set #rank in the query; you don't need a separate statement.
The GROUP BY makes no sense, because you have many unaggregated columns in the SELECT.
If I had to speculate, the root cause of your problems is the missing JOIN condition, and you've put a lot of effort into working around that.