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Re^4: Multiple SQL statements in DBI

by tel2 (Pilgrim)
on Sep 07, 2012 at 22:59 UTC ( #992402=note: print w/replies, xml ) Need Help??

in reply to Re^3: Multiple SQL statements in DBI
in thread Multiple SQL statements in DBI

Thanks for that runrig, but why do you say "I don't think DBI prevents anything", when as chacham has pointed out below, the 'DBI documentation states "Multiple SQL statements may not be combined in a single statement handle ($sth), although some databases and drivers do support this (notably Sybase and SQL Server)."'.  To me, that means, regardless of whether the database driver supports it, DBI will prevent it.  Or am I misunderstanding something?

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Re^5: Multiple SQL statements in DBI
by mje (Curate) on Sep 07, 2012 at 23:59 UTC

    I don't believe DBI prevents any SQL. As far as I am aware prepare is a method in the individual DBDs. I don't believe that quote from the DBI pod has anything to do with SQL injection.

    Just a few reasons why multiple SQL statements in the same prepare/do call are problematic (or unsupportable as is):

    Some DBDs "parse" (mostly roughly) the SQL passed to prepare in order to try and identify placeholders. e.g., DBD::ODBC does this but only because DBI says placeholders may be specified with :name or ? and ODBC itself only defines the latter. I doubt vey much that of those DBDs which parse the SQL to find placeholders many (if any) support recognising N bits of SQL separated by ; So imagine if you did "insert into mytable values(?);insert into mytable values(?)" then any DBD which parses the SQL looking for placeholders may be fooled into thinking you need 2 but in actual fact the database probably executes these sepately.

    Generally speaking the databases which support multiple statements in the same prepare run them individually and often there is something required to make it move from one to the next. Imagine 2 select stmts for differing numbers of columns - you need to stop in between them. Imagine an insert followed by another insert (your example), what is to be returned by execute when it is defined as returning the number of affected rows.

    Then there are batch statements like execute_for_fetch and execute_array. How is the tuple status to be returned for multiple statements.

    There are just too many things which don't fit.

      OK - thanks mje.  I'm with ya.  Thanks for your time.

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