MySQL IF EXISTS not working - mysql

Question

Trying to use IF EXISTS to automatically choose which table to select a record from.
I just get the following error message with the code below.
Where am I going wrong?
IF EXISTS (SELECT 1 FROM Users WHERE Reference='USR00000007')
SELECT * FROM Users WHERE Reference='USR00000007'
ELSE
SELECT * FROM Staff WHERE Reference='USR00000007';
Error: #1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IF EXISTS (SELECT 1 FROM Users WHERE Reference='USR00000007') SELECT * FROM' at line 1

Answer 1

The problem is in MySQL. The IF -- as control flow -- only works in programming blocks such as stored procedures, stored functions, and triggers.
Assuming the columns are the same in both tables, you can do what you want as a single query:
SELECT u.*
FROM Users u
WHERE u.Reference = 'USR00000007'
UNION ALL
SELECT s
FROM Staff s
WHERE s.Reference = 'USR00000007' AND
NOT EXISTS (SELECT 1 FROM Users u2 WHERE u2.Reference = 'USR00000007')

Answer 2

if you want union of two query, both mast have same number of fields (if you need, you can add null fields to query) and fields should have similar type, so try this:
SELECT Users.id as user_id, null as staff_id, some_other_number_field, some_other_char_field
FROM Users
WHERE Reference='USR00000007'
SELECT null , Staff.id, some_other_number_field, some_other_char_field
FROM Staff
WHERE Reference='USR00000007'
and not EXISTS (SELECT 1 FROM Users WHERE Reference='USR00000007');
maybe you need query like this, to always get only users for Reference:
SELECT *
FROM Users
WHERE (Reference='USR00000007'
or exists (
SELECT 1
FROM Staff
WHERE Reference='USR00000007'
and Staff.user_id = Users.id));
but you must have join condition similar to "and Staff.user_id = Users.id"