Question

Say I have a table for students attending school like this:

St_ID |St_Name|Class_ID|Year
------+-------+--------+----
 02345|John   |     345|2011
 07871|Jane   |     234|2010

and so on.
I wish to run queries on each Class which I can get by concatenating Class_ID & Year.
i.e 345-2011 is one class

How do I do count(*), by having the concatenated string in the Group by query?

Answer 1

Two options:

1) Add the concatenated column to the group by clause

Eg.

SELECT columna || columnb || columnc
FROM table
GROUP BY columna || columnb || columnc

2) Add each column that apart’s the concatenated column in the group by seperately

Eg.

SELECT columna || columnb || columnc
FROM table
GROUP BY columna, columnb, columnc