Combining MySQL Queries to get total amount

Question

I'm trying to combine the following 2 queries. table_A stores a list of available appointments and number_open is how many open slots there are for a specific appointment, in this case 2020-06-30 AM which has 5 open. table_B contains any dates waiting to be accepted and has a different record for each date request, so in this case, 2 rows sitting there, 2 dates waiting to be accepted. What I'm trying to do is combine both queries to get a list of how many appointments there are left for each slot, so expected results would be table_A.numberopen (5) minus table_B.Count() (2), result = 3 left to allocate. Everything I try results in duplicate records being returned, I can't seem to narrow it down to just a single total number for that appointment. table_A will always have a record, but table_B may not have any matching records. Thanks in advance for any help you can provide.

SELECT      whendate, slot, initials, numberopen
FROM        table_A
WHERE       whendate = '2020-06-30'
AND         slot = 'AM'
AND         numberopen > 0

SELECT    initials 
FROM      table_B
WHERE     date_requested = "2020-06-30"
AND       ampm_requested = 'AM' 

Answer 1

Your question is quite unclear about the join conditions between the two tables, however I suspect that a correlated subquery would do what you want:

select      
    a.whendate, 
    a.slot, 
    a.initials, 
    a.numberopen - (
        select count(*)
        from table_b b 
        where b.date_requested = a.whendate and b.ampm_requested = a.slot
    ) remaining_open
from table_a a
where a.whendate = '2020-06-30' and a.slot = 'AM'