What kind of boolean/conditionals can I evaluate in SQL/PL?

Some of the conditional statements are pretty obvious and well documented, like "select * from table where day='1-may-2002'", but I would like to know what other boolean expressions can be evaluated. For instance: how do I "say" this in SQL/PL:

   if (table 'payroll' exists) then do whatever    if (there is an index on table xyz) then drop index

Some of my scripts fail because they are trying to do a select on a group of tables (some of which may not exist), or because the script is trying to drop an index which has been dropped already.

TIA, -Ramon F. Herrera Received on Sat May 25 2002 - 13:54:30 CDT