Friday, March 26, 2010

One more SQL hack (or How I Married a Genius)

Here's my problem.  I'm making the following SQL call from my application:

select a.*, b.* from foo a, bar b where a.[column]= b.[column]

I don't know on the application end what fields are in what table, and I don't know what field the join will be on -- that's all dynamic.

I need to know from looking at my result set which table each column came from.  The problem of course is that I can't (at least in mysql) because if you run a select with a *, you only get back column names as a headers.

After beating my head against it, and not coming up with a solution, I IMed my big brain DB team, Greg and Ellie, but they were both apparently at lunch.  Then I remembered at my new job we have a $50k/yr contract with MySQL for support, so I called them.

It took quite a while to explain to the 1st level of customer support what the problem was.  He kept insisting the answer was to use an "as", but I tried that long before I called, and it doesn't work. I had tried:

select a.* as foostuff, b.* as barstuff from foo a, bar b where a.[column]= b.[column]

in the hopes I would get back,, etc etc etc..  No joy.  It still gives you the same tableless headers.  Eventually 1st level customer support kicked it upstairs.  2nd level customer support called me within 1 minute of that happening.  Very impressive.

"It can't be done,"  He let me know.  "When you use the * you are telling the DB to not include table names."  Yes yes, I could use the metadata or information_schema to get around this, but I didn't want to do that because of all the extra junk it involves.

So I resigned myself to having to preload the metadata in the app, and write out more detailed queries.

Fortunately I married a genius, who having finally come back from lunch, within 5 minutes of me explaining the problems said

"select a.*, 1 as NEWTABLE, b.* from foo a, bar b where a.[column]= b.[column]"

GENIUS!  Now I know everything in the result set to the left of the NEWTABLE is a foo, and everything to the right is a bar!!!  GENIUS!!!

And considering I spent (much) less than $50k on the ring 8 years ago, is a real bargain in customer support!

1 comment :

--- said...

That's hilarious and a nice hack ;)