Mysql query Sum with Case

Question

I have two tables.

One contains default data for 3 columns, (value is 1 or 0) In the same table is a ClientID

Another table contains edited data with a date, ClientID and a extra column named 'Changed'

If Changed = 1 then the values in the 3 columns are changed and therefore need to be read from the second table.

This al works fine, but I want to make a report in php where a daterange can be selected and a query should group by ClientID and count all 1's in the selected daterange of the 3 columns. (Each column seperate)

Here's the trick: When Changed = 0 in the specific row then It should check the default value and if Changed = 1 it should check the second table. And then count it with the previous rows.

I hope you understand what I want to create

Answer 1

You can use IF function from SQL

For example

SELECT SUM(IF(Changed=1, t1.col1, t2.col2)) FROM t1, t2 WHERE t1.id=t2.id

This is example how you can use columns for case in SUM