SQL Count dates in one table that occur between a start and end date in another table

Question

I have a table ' tb1 ' that looks like this:

Nurse    Start          End        Posting 
---    ------------   -----------  -------
123    2012-04-26     2014-02-10   ABC
123    2015-03-21     2016-01-12   DEF
123    2016-05-19     2018-09-14   MNO
256    2017-01-15     2018-02-20   JKL
256    2018-07-13     2019-01-17   MNO

And another separate table ' tb2 ' that can be linked to the above table by the column Nurse .

Nurse    PartyDate 
---    ------------  
123    2017-02-23     
123    2017-05-11     
256    2018-11-28

I want to count the number of parties that each Nurse attends between the start and end date of each posting. The expected result should look like this:

Nurse    Start          End        Posting   Count
---    ------------   -----------  -------   ------
123    2012-04-26     2014-02-10    ABC        0
123    2015-03-21     2016-01-12    DEF        0
123    2016-05-19     2018-09-14    MNO        2
256    2017-01-15     2018-02-20    JKL        0
256    2018-07-13     2019-01-17    MNO        1

Answer 1

One method is a correlated subquery:

select t1.*,
       (select count(*)
        from tb2 t2
        where t2.nurse = t1.nurse and
              t2.partydate >= t1.start and
              t2.partydate <= t1.end
       ) as num_parties
from tb1 t1;

For performance, you want an index on tb2(nurse, start, end) .

Answer 2

Here is another way of achieving this.

select tb1.nurse, tb1.start, tb1.end, tb1.posting, count(tb2.partydate) Count
from tb1 left join tb2 on (tb1.nurse = tb2.nurse 
                           and tb2.partyDate between tb1.start and tb1.end)
group by tb1.nurse, tb1.start, tb1.end, tb1.posting 
;

It is expected to run faster than the correlated queries as in the case of latter for every record from tb1 DB engine will have to scan through tb2 table