Joining a group by subquery with substr columns - sql

Question

I am trying to join a group by subquery with another table but am running in to issues since the subquery contains aliases which are only valid within the subquery. The issue is that the AREA table only has the 3 digit locker id embedded as part of a longer string in the ARE_CODE column which is why I am using substr. Here is the group by subquery:
SELECT SUBSTR(ARE_CODE,0,3) AS "Locker", COUNT(ARE_UID) AS "Qty"
FROM AREA
WHERE SUBSTR(ARE_CODE,0,1)='L'
GROUP BY SUBSTR(ARE_CODE,0,3)
It runs fine on its own - the problem is that I am trying to join this with another table in the main query. The alias is only valid within the subquery itself and can't be referenced as I did below. I have also tried joining the substr function directly but that doesn't work either.
SELECT P.Locker, P.Permit, L.Qty
FROM PERMITS P
INNER JOIN (SELECT SUBSTR(ARE_CODE,0,3) AS "Locker", COUNT(ARE_UID) AS "Qty"
FROM AREA_LKP
WHERE SUBSTR(ARE_CODE,0,1)='L'
GROUP BY SUBSTR(ARE_CODE,0,3) L ON L.Locker = P.Locker
Any suggestions on how to work around this issue?

Answer 1

You just need to remove the double quotes from the alias as they makes the alias case sensistive and you must have to use them with double quotes wherever you want to use them in your query with exact case.
So better not to use the double quotes for alias untill and unless you want to give the case sensitive name or want space in the alias.
SELECT P.Locker, P.Permit, L.Qty
FROM PERMITS P
INNER JOIN (SELECT SUBSTR(ARE_CODE,0,3) AS Locker, COUNT(ARE_UID) AS Qty
FROM AREA_LKP
WHERE SUBSTR(ARE_CODE,0,1)='L'
GROUP BY SUBSTR(ARE_CODE,0,3) L ON L.Locker = P.Locker

Answer 2

The issue are the double quotes. But your query has other eccentricities:
In strings, the first character is at 1, not 0 (although 0 is allowed).
LIKE is a good way to do string comparisons.
So:
SELECT P.Locker, P.Permit, L.Qty
FROM PERMITS P INNER JOIN
(SELECT SUBSTR(ARE_CODE, 1, 3) AS Locker, COUNT(*) AS Qty
FROM AREA_LKP
WHERE ARE_CODE LIKE 'L%'
GROUP BY SUBSTR(ARE_CODE, 1, 3)
) L
ON L.Locker = P.Locker;
I should note that LIKE permits the use of an index on the column, but SUBSTR() generally prevents that.

Answer 3

Why you`re set the alias of the table at the end of the subquery?
INNER JOIN (SELECT SUBSTR(ARE_CODE,0,3) AS "Locker", COUNT(ARE_UID) AS "Qty"
FROM AREA_LKP
WHERE SUBSTR(ARE_CODE,0,1)='L'
GROUP BY SUBSTR(ARE_CODE,0,3) L
Try:
INNER JOIN (SELECT SUBSTR(ARE_CODE,0,3) AS "Locker", COUNT(ARE_UID) AS "Qty"
FROM AREA_LKP L
WHERE SUBSTR(ARE_CODE,0,1)='L'
GROUP BY SUBSTR(ARE_CODE,0,3)
For sharing expressions between different statements take a look at the with-statement:
https://docs.oracle.com/cd/E17952_01/mysql-8.0-en/with.html
Finally, are you sure that:
L.Locker = P.Locker
Can be evaluated as expected? So is the substringged value of L.Locker same as P.Locker?