Question

I need to flatten out 2 rows in a vertical table (and then join to a third table) I generally do this by making a derived table for each field I need. There’s only two fields, I figure this isn’t that unreasonable.

But I know that the rows I want back in the derived table, are the subset that’s in my join with my third table.
So I’m trying to figure out the best derived tables to make so that the query runs most efficiently.

I figure the more restrictive I make the derived table’s where clause, the smaller the derived table will be, the better response I’ll get.

Really what I want is to correlate the where clause of the derived table with the join with the 3rd table, but you can’t do that in sql, which is too bad. But I’m no sql master, maybe there’s some trick I don’t know about.

The other option is just to make the derived table(s) with no where clause and it just ends up joining the entire table twice (once for each field), and when I do my join against them the join filters every thing out.

So really what I’m asking I guess is what’s the best way to make a derived table where I know pretty much specifically what rows I want, but sql won’t let me get at them.

An example:

table1
------

id  tag     value
--  -----   -----
1   first   john
1   last    smith
2   first   sally
2   last    smithers


table2
------

id     occupation
--     ----------
1      carpenter
2      homemaker


select table2.occupation, firsttable.first, lasttable.last from
table2, (select value as first from table1 where tag = 'first') firsttable,
(select value as last from table1 where tag = 'last') lasttable
where table2.id = firsttable.id and table2.id = lasttable.id

What I want to do is make the firsttable where clause where tag=’first’ and id = table2.id

Answer 1

I think what you’re asking for is a COMMON TABLE EXPRESSION. If your platform doesn’t implement them, then a temporary table may be the best alternative.