Question

I’m trying to use a CASE statement on a messages table. I need to mark a message as viewed when the first CASE statement is true.

UPDATE messages
SET to_del=CASE WHEN to=2 THEN 1 ELSE to_del END,
    from_del=CASE WHEN from=2 THEN 1 ELSE from_del END,
WHERE company_id = '1'

Here is before employee 2 deletes messages:

company_id | to | from | to_del | from_del | viewed
    1        4     2       0         0         0
    1        2     4       0         0         0

Here is after employee 2 deletes messages:

company_id | to | from | to_del | from_del | viewed
    1        4     2       0         1         0    
    1        2     4       1         0         0    <-- viewed should be 1

Trying this didn’t work, but it should be something similar in the same query:

UPDATE messages
SET to_del=CASE WHEN to=2 THEN 1 ELSE to_del, viewed=1 END, <-- viewed=1 not working
    from_del=CASE WHEN from=2 THEN 1 ELSE from_del END,
WHERE company_id = '1'

How can I make viewed=1 only when the 1st CASE statement is true?

Answer 1

If the logic is really that simple, is there any reason not to go for the straight forward solution;

UPDATE messages
SET to_del =   if( to=2, 1, to_del ), 
    viewed =   if( to=2, viewed, 1 ),
    from_del = if( from=2, 1, from_del )
WHERE company_id = '1'

or, if you really want to eliminate the common subexpression (a bit overkill for to=2)

UPDATE messages
SET to_del =   if( @tmp:=to=2, 1, to_del ), 
    viewed =   if( @tmp, viewed, 1 ),
    from_del = if( from=2, 1, from_del )
WHERE company_id = '1'