SQL SELECT, GROUP BY

Question

Why do I have to group all my selected columns in group by if i started to group by one column?

For instance, why can't i just have only two groups? as below:

SELECT A, B, C, D
FROM MYTB
GROUB BY A, B

Answer 1

You can group by as many elements as you want. Just make sure any non-grouped elements in your select have some aggregation.

SELECT A, B, SUM(C), MAX(D)
FROM MYTB
GROUP BY A, B

See

• Basic info: http://www.techonthenet.com/sql/group_by.php
• Grouping by more than 1 column: http://www.w3schools.com/sql/trysql.asp?filename=trysql_select_groupby_2