How to declare a cursor if table used in select statement may not exist sometimes? - sql

Question

I need to run below code on different environments and it works fine when table1 exists but when it does not exist then it throws error in cursor declaration that "table or view does not exist".
I am running this on Oracle.
Could you please help me in correcting this?
Thanks in advance.
DECLARE
CURSOR my_cursor IS (select "col1" from "table1");
name1 VARCHAR2(256);
tableCount NUMBER;
BEGIN
Select count(*) into tableCount from user_tab_cols where table_name = 'table1' and column_name = 'col2';
IF tableCount > 0 THEN
OPEN my_cursor;
LOOP
FETCH my_cursor into name1;
EXIT WHEN my_cursor%notfound;
-- Update or delete statement here
DBMS_OUTPUT.PUT_LINE('value is ' || name1);
END LOOP;
CLOSE my_cursor;
END IF;
END;
/

Answer 1

A CURSOR with a defined return type is strongly typed. Sys_refcursors are weakly typed. This means that any return type in a CURSOR must be valid. A SYS_REFCURSOR is more flexible and can be defined at a later time.
When setting a CURSOR in the declaration section, you must use a SQL statement that will execute properly. In this case, you are setting the CURSOR to select a column from a table that does not exist. The database execution will not reach the body of the code since it errors prior to exiting the declaration block.
To fix this, use a SYS_REFCURSOR with a dynamic sql query, as mentioned by Tejash above. This allows you to check to see if the table exists before setting the cursor. If the table exists, set the cursor to select from the specified table. If it doesn't, output a message saying that it does not exist.
Note that you can also use the SQL error codes, as shown in other answers. I prefer personally to handle business rules in the logic prior to the error occurring.
DECLARE
my_cursor sys_refcursor;
name1 VARCHAR2(256);
tableCount NUMBER;
BEGIN
Select count(*) into tableCount from user_tab_cols where table_name = 'table1' and column_name = 'col2';
IF tableCount > 0 THEN
OPEN my_cursor for 'select order_id from table1';
LOOP
FETCH my_cursor into name1;
EXIT WHEN my_cursor%notfound;
-- Update or delete statement here
DBMS_OUTPUT.PUT_LINE('value is ' || name1);
END LOOP;
CLOSE my_cursor;
else
dbms_output.put_line('Table does not exist');
END IF;
END;
http://docs.oracle.com/database/122/LNPLS/static-sql.htm#LNPLS568

Answer 2

You can use the exceptions and dynamic string for a cursor as follows:
DECLARE
MY_CURSOR SYS_REFCURSOR;
NAME1 VARCHAR2(256);
TABLECOUNT NUMBER;
BEGIN
OPEN MY_CURSOR FOR 'SELECT ACC_NR FROM ACCOUNT'; -- dynamic string for cursor
LOOP
FETCH MY_CURSOR INTO NAME1;
EXIT WHEN MY_CURSOR%NOTFOUND;
DBMS_OUTPUT.PUT_LINE('value is ' || NAME1);
END LOOP;
CLOSE MY_CURSOR;
EXCEPTION -- exception handling using SQLCODE
WHEN OTHERS THEN
IF SQLCODE = -942 THEN
DBMS_OUTPUT.PUT_LINE('Table does not exists');
ELSIF SQLCODE = -904 THEN
DBMS_OUTPUT.PUT_LINE('Invalid column name');
ELSE
DBMS_OUTPUT.PUT_LINE('Other error');
END IF;
END;
/