Question

I need to select data from two table using a join. This is fairly simple and have no problems here. The problem occurs when the field I am joining is used as two separate foreign keys (I didn’t design this). So the ID field that I join on is either a positive or negative number.

If it’s a positive number it relates to ID_1 on the table_2 table, if it’s a negative, the number relates to ID_2 on the table_2 table. However the ID_2 will be a positive number (even though it’s stored as a negative in the foreign key). Obviously there are no constraints to enforce these – so in essence not real foreign keys :/

The SQL I’m using goes something like this and is fine for the positive numbers:

select t1.Stuff, t2.MoreStuff from table_1 t1
join table_2 t2 on t1.ID_1 = t2.ID_1
where ...

How to incorporate the negative aspect of this into the join. Is this even possible? Ideally I’d like to alter the table to my needs but apparently this is not a valid option. I’m well and truly stuck.

The only other idea I’ve had is a separate sql statement to handle these odd ones. This is all being run by clr sql from C#. Adding a separate SqlCommand to the code will most likely slow things down hence why I’d prefer to keep it all in one command.

Your input is welcome, thanks 🙂

Answer 1

Let’s say the tables look like this:

Table1 (id INT, foo INT, fk INT)

Table2 (id1 INT, id2 INT, bar VARCHAR(100))

…where fk can be used to look up a row in Table2 using id1 if positive and id2 if negative.

Then you can do the join as follows:

SELECT T1.id, T1.foo, T2.bar
FROM Table1 T1 INNER JOIN Table2 T2
ON    (T1.fk > 0 AND T2.id1 = T1.fk)
   OR (T1.fk < 0 AND T2.id2 = - T1.fk)