Selecting across multiple tables with UNION

问题

I reworked my database from one user table to multiple user tables (divided per role): tblStudents, tblTeachers, tblAdmin

When logging in, I didn't want to run three queries to check if the user exists somewhere in my DB. So what I did was put together the following query with union

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

This selects the fields that are the same across all tables and outputs the results nicely for me.

So, I decided to try this and for some reason, my login form wouldn't work. For my login form, I added a where clause which checks for the email address. I ran the query in my database app and surprisingly, when I do for example where email = "admin@admin.be" (this email exists in my database tblAdmin), it also selects a record from my students table.

With the where clause:

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
where email = "admin@admin.be"

The records both have id = 1 but I don't understand why it would select the student record when I'm filtering on the admin email address. Why is this? Can someone explain and provide me with a better solution to this problem? I basically have one login form and need to select across multiple tables to check if the user exists in my db.

回答1:

Thanks for updating the query; now we can see that the WHERE condition is only applied to the last UNIONed query. You need to either add that WHERE clause to each query, or wrap it as a subselect and apply the WHERE clause to that.

select s.id as id, s.email as email, s.password as password, s.role as role from tblStudents s
where email = "admin@admin.be"
union
select a.id as id, a.email as email, a.password as password, a.role as role from tblAdmin a
where email = "admin@admin.be"
union
select t.id as id, t.email as email, t.password as password, t.role as role from tblTeachers t
where email = "admin@admin.be"

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 = "admin@admin.be"

回答2:

SELECT
    R.co_C,
    company3.Statuss
FROM
    (
        SELECT
            company1.co_C
        FROM
            company1
        UNION ALL
            SELECT
                company2.co_C
            FROM
                company2
    ) AS R
LEFT JOIN company3 ON R.co_C = company3.co_A;

来源：https://stackoverflow.com/questions/6066197/selecting-across-multiple-tables-with-union

标签

mysql

select

union