[Zope] <dtml-if> errors in sql method

[Tim Hicks]

<dtml-if "which_table == 'Plays'">

SELECT *
FROM <dtml-var which_table>
WHERE "Play title" like <dtml-sqlvar
"'%'+_.string.strip(form_string)+'%'" type=string>;

Can anybody explain to me what is going on with my zsql method?  Zope
gives me the following error when I try and change my method to what
is above.

Invalid attribute name, "which_table", for tag <dtml-if "which_table
== 'Plays'">, on line 16 of <string>

I've scoured the mailing list, but I'm still mightily confused.
which_table is one of the arguments for the the sql method and Plays
is a string that I want to compare it with.  which_table comes from a
form elsewhere.  I'm sure it's something really simple, and I'm sure
it's been discussed here before (at length?), but ATM, I don't even
know what I'm looking for!

Thanks for the help

tim