Select rows from table B if it is less than Table B Column col1 comparing Table A

Question

Table A

UHID    SlotID       Date
111      1          2019-12-22 
222      1          2019-12-22 
333      3          2019-12-22 
444      1          2019-12-22 
555      2          2019-11-25 
666      3          2019-12-22 

Table B (Slot)

ID  Slot         Max slot
1   Morning       3
2   Noon          5
3   Evening       2
4   Night         5

Now I want to get available slot from table B on date 2019-12-22 and less than Max

the result will look like Table B (Slot)

ID  Slot         Max slot
2   Noon          5
4   Night         5

Thanks

Answer 1

You will need a LEFT JOIN here and GROUP BY the table B rows. The COUNT condition then goes to the HAVING clause.

select b.*
from table_b b
left join table_a a
  on  a.SlotID = b.ID
  and a.Date = '2019-12-22'
group by b.ID
having count(a.SlotID) < b.Max_slot

Result:

| ID  | Slot  | Max_slot |
| --- | ----- | -------- |
| 2   | Noon  | 5        |
| 4   | Night | 5        |

View on DB Fiddle

Note: Depending on your server version, settings and keys you might need to include all columns from the table B in the GROUP BY clause.

group by b.ID, b.Slot, b.Max_slot

You will find more info in the docs .

Answer 2

I think you might be able to benefit from a CTE:

WITH slot_counts (slot_id, slot_count)
  AS (SELECT slotid, count(*) FROM `tableA` WHERE `date` = '2019-12-22' GROUP BY slotid)
SELECT *
  FROM `tableB`, slot_counts
 WHERE `max slot` < slot_counts.slot_count
   AND tableB.slot = slot_counts.slot_id;