Statements in if statement with false condition are executed - sql

Question

I have created this guard function:
create or replace
FUNCTION checkTableExists (tableName varchar2)
RETURN BOOLEAN
IS c INT;
BEGIN
SELECT COUNT(*) INTO c FROM user_tables where table_name = upper(tableName);
return c = 1;
END;
I try to use it like:
IF checkTableExists ('NO_TABLE') THEN
DELETE FROM NO_TABLE;
END IF;
Even though the table doesn't exist, I get:
Error report:
ORA-06550: line 6, column 17:
PL/SQL: ORA-00942: table or view does not exist
ORA-06550: line 6, column 5:
PL/SQL: SQL Statement ignored
06550. 00000 - "line %s, column %s:\n%s"
*Cause: Usually a PL/SQL compilation error.
*Action:
How do I get around this? Dynamic SQL?
UPDATE: I get no error if I run:
IF checkTableExists ('NO_TABLE') THEN
dbms_output.put_line('argh');
END IF;
And argh is not output. If I run the above with a table that does exist, argh is output as expected.

Answer 1

Your statement is parsed completly. So if you replace
DELETE FROM NO_TABLE;
by
null;
your statement should work.
Your error message was a little bit misleading because in line 6 is the select on user_tables and the delete is in line 2 in a different statement. This makes it harder to debug.
So you have to use dynamic sql:
execute immidiate 'delete from ' || 'NO_TABLE';

Answer 2

Don't use a guard function; just try to delete the table and catch the exception if it does not exist:
DECLARE
table_not_exists EXCEPTION;
PRAGMA EXCEPTION_INIT( table_not_exists, -942 );
BEGIN
EXECUTE IMMEDIATE 'DELETE FROM no_table';
DBMS_OUTPUT.PUT_LINE( 'deleted' );
EXCEPTION
WHEN table_not_exists THEN
DBMS_OUTPUT.PUT_LINE( 'did not exist' );
END;
/
db<>fiddle
If you don't want to initialise the exception in lots of different PL/SQL blocks then initialise once it in a package:
CREATE PACKAGE exceptions IS
table_not_exists EXCEPTION;
PRAGMA EXCEPTION_INIT( table_not_exists, -942 );
END;
/
and the code is then simply:
BEGIN
EXECUTE IMMEDIATE 'DELETE FROM no_table';
DBMS_OUTPUT.PUT_LINE( 'deleted' );
EXCEPTION
WHEN EXCEPTIONS.table_not_exists THEN
DBMS_OUTPUT.PUT_LINE( 'did not exist' );
END;
/
db<>fiddle
If you do want to use your guard function (I'd advise that you don't) then just use EXECUTE IMMEDIATE:
IF checkTableExists ('NO_TABLE') THEN
EXECUTE IMMEDIATE 'DELETE FROM NO_TABLE';
END IF;