Question

I am doing a join between two tables and want to select the columns based on whether they have a record or not. I’m trying to avoid having multiple of the same field and am trying to condense them into single columns. Something like:

Select 
    id = (CASE WHEN a.id IS NULL THEN b.id ELSE a.id END),
    name = (CASE WHEN a.name IS NULL THEN b.name ELSE a.name END)
From Table1 a
Left Join Table2 b
On a.id = b.id
Where a.id = @id

I’d like id to populate from Table1 if a record exists, but if not pull from Table2. The previous code returns no records because there are no NULL values in Table1 so my question is how do I run a check to see if any records even exist? Also if anyone knows of a better way to accomplish what I am trying to do I appreciate guidance and constructive criticism.

EDIT

It looks like COALESCE will work for what I’m trying to accomplish. I’d like to give a little more info on exactly what I am working with and get some advice on whether I am using the best method.

I have a bloated table Table2 and it is in production. I’m working on building new web applications for this system but can’t justify a complete database redesign so I am trying to do one “on the fly”. I’ve created a new table Table1 and I am writing stored procedures for the following methods Get(Select), Set(Update), Add(Insert), Remove(Delete). This way, to my code, it will seem that I am working with a single table that is not bloated. My code will simply call one of the SP methods and then the stored procedure will handle the data between the old table and the new. I am currently working on the Get method and I need to check the old table Table2 for a record if it doesn’t exist in Table1.
Thanks to the suggestions here my query currently looks like this:

Select
    id = coalesce(a.id, b.student_number),
    first_name = coalesce(a.first_name, b.first_name),
    last_name = coalesce(a.last_name, b.last_name),
    //etc
From Table1 a
Full Outer Join Table2 b
On a.id = b.student_number
Where (a.id = @id Or b.student_number = @id)

This works for what I’m trying to accomplish, I’d like to throw it out there to the experienced crowd for any tips or suggestions if there are better or more correct ways to go about this.

Thanks

Answer 1

I suspect your problem may come from doing a left join. Try again using a full outer join, like this:

Select 
    id = coalesce(a.id, b.id),
    name = coalesce(a.name, b.name)
From Table1 a
full outer Join Table2 b
On a.id = b.id
Where a.id = @id