优化 SQL Select 与 Union

Question

I have the following SQL query, in which I make use of UNIONs:

SELECT notification FROM NotificationDb notification WHERE notification.proc_name = 'a' AND notification.status = 1 AND notification.module = 1
UNION
SELECT notification FROM NotificationDb notification WHERE notification.proc_name = 'b' AND notification.param1 >0 AND notification.status = 1 AND notification.module = 1
UNION
SELECT notification FROM NotificationDb notification WHERE notification.proc_name = 'b' AND notification.param1 <= 0 AND notification.status = 1 AND notification.module = 1

I have it set up this way, because I need them ordered by proc_name field and also param1 value for cases where proc_name is 'b'

QUESTION: Is there a better, a more efficient way of putting together a query that would return the same result?

Any help would be appreciated!

Answer 1

If I understand correctly, you don't need unions here, you can just do a single distinct select:

SELECT DISTINCT notification
FROM NotificationDb
WHERE status = 1 AND module = 1 AND proc_name IN ('a', 'b')
ORDER BY
    proc_name,
    param1 DESC;

Here, we add an ORDER BY clause which puts a proc_name records before b . Then, within all b records, we show larger param1 records first. This sorting maintain the perceived ordering you have in current union query (though it should be noted that no such actual order is there; always use an ORDER BY clause if you expect a certain sorting in your result set).