Question

I have the following table:

Col1  Col2
A     ...
A     ...
A
B
B
C
D

I want to be able to consider A & B as if they were one for the purposes of group by, so that

select Col1, count(*) from table group by Col1

will return

Col1  count(*)
AB    5
C     1
D     1

Answer 1

Group on a conditional column via a CASE statement:

SELECT
  CASE WHEN Col1 IN ('A','B') THEN 'AB' ELSE Col1 END AS Col1Group,
  COUNT(*)
FROM table
GROUP BY
    CASE WHEN Col1 IN ('A','B') THEN 'AB' ELSE Col1 END

You did not specify your RDBMS, but some (like MySQL) will permit you to use the alias Col1Group inside the GROUP BY rather than having to duplicate the whole expression down there.