Apocalisp Apocalisp - 1 year ago 66
SQL Question

Is it possible to use GROUP BY with bind variables?

I want to issue a query like the following

select max(col1), f(:1, col2) from t group by f(:1, col2)

is a bind variable. Using
, if I say

("select max(col1), f(?, col2) from t group by f(?, col2)")

I get an error from the DBMS complaining that
f(?, col2)
is not a GROUP BY expression.

How does one normally solve this in JDBC?

Answer Source

I suggest re-writing the statement so that there is only one bind argument. This approach is kind of ugly, but returns the result set:

select max(col1) 
     , f_col2
  from (
         select col1
              , f(? ,col2) as f_col2 
           from t
    by f_col2

This re-written statement has a reference to only a single bind argument, so now the DBMS sees the expressions in the GROUP BY clause and the SELECT list are identical.



(I wish there were a prettier way, this is why I prefer the named bind argument approach that Oracle uses. With the Perl DBI driver, positional arguments are converted to named arguments in the statement actually sent to Oracle.)

I didn't see the problem at first, I didn't understand the original question. (Apparently, several other people missed it too.) But after running some test cases, it dawned on me what the problem was, what the question was working.

Let me see if I can state the problem: how to get two separate (positional) bind arguments to be treated (by the DBMS) as if it were two references to the same (named) bind argument.

The DBMS is expecting the expression in the GROUP BY to match the expression in the SELECT list. But the two expressions are considered DIFFERENT even when the expressions are identical, when the only difference is that each expression references a different bind variable. (We can demonstrate some test cases that at least some DBMS will allow, but there are more general cases that will raise an exception.)

At this point the short answer is, that's got me stumped. The suggestion I have (which may not be an actual answer to the original question) is to restructure the query.


I can provide more details if this approach doesn't work, or if you have some other problem figuring it out. Or if there's a problem with performance (I can see the optimizer choosing a different plan for the re-written query, even though it returns the specified result set. For further testing, we'd really need to know what DBMS, what driver, statistics, etc.)

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