mysql JOIN ON IF()? - sql

Question

I am trying to use sql such as this:
SELECT t.*, t2.* FROM templates t
LEFT JOIN IF(t.t_type = 0,'templates_email',
IF(t.t_type = 1,'templates_sms','templates_fax')) t2
ON t.t_id = t2.t_id;
Is it possible to do something like that?
basically I want to join on one of three tables based on the value from the row.
Is this recommended if it is possible?
update
So,
basically the templates table is the table that contains all the information that every template must have, eg name, id, description
then you have the templates_x tables these tables contain the fields that are unique to each template type.
(There are quite a few and having a single table with null fields for those not applicable is not practical).
The tables are called templates_x however the appropriate x is stored in the templates table as an int flag.
The join between the templates_x tables and the templates table is through the t_id.
So what do you suggest?

Answer 1

Is it possible to do something like that?
No, you can't use dynamically assigned tables (join or otherwise) without using dynamic SQL syntax (See MySQL's PreparedStatement syntax).
This non-dynamic re-write of your pseudo-query assumes that the three template tables you join to all have the same number of columns, and the same data types:
SELECT t.*,
te.*
FROM TEMPLATES t
JOIN TEMPLATES_EMAIL te ON te.t_id = t.t_id
WHERE t.t_type = 0
UNION
SELECT t.*,
ts.*
FROM TEMPLATES t
JOIN TEMPLATES_SMS ts ON ts.t_id = t.t_id
WHERE t.t_type = 1
UNION
SELECT t.*,
tf.*
FROM TEMPLATES t
JOIN TEMPLATES_FAX tf ON tf.t_id = t.t_id
WHERE t.t_type NOT IN (0, 1)

Answer 2

Instead of a conditional join, you can join the data then select which field you want.
Example:
SELECT
CASE
WHEN t.type = 0 THEN [templates_email]
WHEN t.type = 1 THEN [templates_sms]
WHEN t.type = 2 THEN [templates_fax]
END AS [selected_field]
FROM
t
LEFT JOIN t1 ON t.t_id = t1.t_id
LEFT JOIN t2 ON t.t_id = t2.t_id
LEFT JOIN t3 ON t.t_id = t3.t_id

Answer 3

I'm afraid it would have to be something like this (if all tables have the same columns):
SELECT t.*, t2.* FROM templates t
LEFT JOIN templates_email t2
ON t.t_id = t2.t_id
WHERE t.t_type = 0
UNION
SELECT t.*, t2.* FROM templates t
LEFT JOIN templates_sms t2
ON t.t_id = t2.t_id
WHERE t.t_type = 1
UNION
SELECT t.*, t2.* FROM templates t
LEFT JOIN templates_fax t2
ON t.t_id = t2.t_id
WHERE t.t_type NOT IN (0,1)
Or like this:
SELECT t.*, t1.*, t2.*, t3.*
FROM templates t
LEFT JOIN templates_email t1
ON t.t_id = t1.t_id
AND t.type_id = 0
LEFT JOIN templates_sms t2
ON t.t_id = t2.t_id
AND t.type_id = 1
LEFT JOIN templates_fax t3
ON t.t_id = t3.t_id
AND t.type_id NOT IN (0,1)
.. and live with all your NULL columns.