Bedankt voor het bijwerken van de vraag; nu kunnen we zien dat de WHERE-voorwaarde alleen wordt toegepast op de laatste UNIONed-query. U moet ofwel voeg die WHERE-component toe aan elke query, of wikkel deze in als een subselect en pas de WHERE-component daarop toe.
select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
where email = "[email protected]"
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
where email = "[email protected]"
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t
where email = "[email protected]"
of
SELECT * FROM (
select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t
) foo where email = "[email protected]"