sql left join for two tables using a CONCAT string as the argument

Question

I need to get a title from table 2, table 2 has title and id column. Table 1 has some data and three of these columns concatenated together makeup the id that can be found in table 1.

I used CONCAT_WS() function and gave this column an alias name and need to use the Alias for the on argument(At least this is what I understood I needed to do)

I thought this could be a simple left join, yet it is not working for me.

This is my query

SELECT
    table_openers.mail,
    table_openers.f_name,
    table_openers.l_name,
    table_openers.Quality,
    CONCAT_WS('-',
            table_openers.esp,
            table_openers.acc,
            table_openers.group) as 't1aid',
    table_groups.aid,
    table_groups.group_name
FROM
    lance_mailstats.table_openers
        LEFT JOIN
    lance_mailstats.table_groups ON table_groups.aid = t1aid;

I get results for mail, f_name, l_name, Quality and t1aid, but the aid and group_name columns of the second table return null.

Answer 1

I feel like you can't use an alias in the ON clause. Try doing

LEFT JOIN
    lance_mailstats.table_groups ON table_groups.aid = CONCAT_WS('-',
            table_openers.esp,
            table_openers.acc,
            table_openers.group);

"You can use the alias in GROUP BY, ORDER BY, or HAVING clauses to refer to the column" (from dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html).

And "The conditional_expr used with ON is any conditional expression of the form that can be used in a WHERE clause" (from dev.mysql.com/doc/refman/5.1/en/join.html).

So as a logical inference you're not allowed to use aliases in ON clauses.

Answer 2

try to use a subquery.. it goes like this.........

ex.

SELECT tbl1.mail, tbl1.f_name, tbl1.l_name,tbl1.Quality, tbl1.t1aid,table_groups.aid, table_groups.group_name

    FROM

       (SELECT
              table_openers.mail,
              table_openers.f_name,
              table_openers.l_name,
              table_openers.Quality,
                CONCAT_WS('-',
                   table_openers.esp,
                   table_openers.acc,
                   table_openers.group) as 't1aid',
             FROM
             lance_mailstats.table_openers )tbl1

       LEFT JOIN
       lance_mailstats.table_groups ON table_groups.aid = tbl1.t1aid;