How use custom alias field from select fields in join? - mysql

Question

How use custom alias field from select fields in join?
SELECT LOWER(CONCAT(`ufn`.`value`, '.', `uln`.`value`, CAST(RAND()*1e6 AS UNSIGNED))) AS `newlogin`
FROM `user_firstname` AS `ufn`
INNER JOIN `user_lastname` AS `uln`
LEFT JOIN `user` AS `u` ON `u`.`login` = `newlogin`
WHERE `ufn`.`sex` = 'male' AND `u`.`id` IS NULL
ORDER BY RAND()
LIMIT 1
Result: sql-error.png
EDIT:
My solution, but, ...:
SELECT #newlogin
FROM (
SELECT #newlogin := LOWER(CONCAT(`ufn`.`value`, '.', `uln`.`value`, CAST(RAND()*1e6 AS UNSIGNED)))
FROM `user_firstname` AS `ufn`
INNER JOIN `user_lastname` AS `uln`
WHERE `ufn`.`sex` = 'male'
ORDER BY RAND()
LIMIT 1
) AS `ufnX`
LEFT JOIN `user` AS `u` ON `u`.`login` = #newlogin
WHERE `u`.`id` IS NULL

Answer 1

Create the new login in a subquery and join it in:
SELECT LOWER(CONCAT(`ufn`.`value`, '.', `uln`.`value`, nl.num)) AS newlogin
FROM (SELECT CAST(RAND()*1e6 AS UNSIGNED))) AS num
) nl CROSS JOIN
`user_firstname` ufn CROSS JOIN
`user_lastname` uln LEFT JOIN
`user` u
ON `u`.`login` = LOWER(CONCAT(ufn.`value`, '.', uln.`value`, nl.num))
WHERE `ufn`.`sex` = 'male' AND `u`.`id` IS NULL
ORDER BY RAND()
LIMIT 1;
Note I replaced the INNER JOIN with CROSS JOIN. INNER JOIN with no ON makes no sense.
EDIT:
You seem to want to test multiple values to get a match. You could try putting the comparison in the HAVING clause. That clause can reference columns in the SELECT:
SELECT LOWER(CONCAT(ufn.`value`, '.', uln.`value`, CAST(RAND()*1e6 AS UNSIGNED))) AS newlogin
FROM user_firstname` ufn CROSS JOIN
`user_lastname` uln
WHERE `ufn`.`sex` = 'male'
HAVING NOT EXISTS (SELECT 1
FROM user u
WHERE u.login = newlogin
)
ORDER BY RAND()
LIMIT 1;