Select from a table which might not exist - oracle

Question

We have an issue that needs a bad hack to get around. Let me give you some context:
We have an app the overrides a customers configuration settings upon uninstalling/reinstalling it. It gets installed with default values, overriding any settings the customer put in.
The solution by management would be to create two scripts, one for each step:
Create a temporary table and copy over the configuration settings into it before uninstalling the app.
Once the app is re-installed, copy over the values from the temporary table back into the original table to retain their settings.
I'm not very fond of their solution, but I have to go with it.
I have step 1 down, but I'm having trouble dealing with the situation of running the second script (step 2) without the first script (step 1) being run before.
In essence, the temporary table would not be there when the second script compiles if someone else in a different department forgets to run the first one.
This is the code I'm currently using for the second script.
DECLARE
lvnTableExists NUMBER(1);
lvbTempTableCopied BOOLEAN;
lvsTempTable VARCHAR2(21) := 'TEMP_TABLE';
BEGIN
-- CalcTypVarValue Table Copy
SELECT COUNT(*)
INTO lvnTableExists
FROM ALL_TABLES x
WHERE x.Table_Name = lvsTempTable ;
IF lvnTableExists = 1 THEN
FOR CalcRow IN (SELECT * FROM Temp_Table) LOOP -- Temp_Table will not exist if first script didn't run, causing a compile error
UPDATE SomeOtherTable c
SET c.foo= CalcRow.foo,
c.bar= CalcRow.bar,
c.DateLastMaint = SYSDATE
WHERE c.bob= CalcRow.bob
AND c.bill= CalcRow .bill;
END LOOP;
lvbTempTableCopied := TRUE;
ELSE
lvbTempTableCopied := FALSE;
END IF;
EXCEPTION
WHEN OTHERS THEN
...
...
My problem is that if Temp_Table doesn't exist at all, then I'll get a compile time error, so the script won't run at all. I need it to run so I can take action on whether to do something else if the table doesn't exist based on lvbTempTableCopied.
I've heard of bypassing it with something like FOR CalcRow IN (EXECUTE IMMEDIATE 'SELECT * FROM ' || lvsTempTable), but I can't use it within a FOR IN LOOP like that.
How would I use EXECUTE IMMEDIATE to bypass the compile time error?

Answer 1

You can do it dynamically using REF CURSOR, see sample code below,
DECLARE
TYPE cur_typ IS REF CURSOR;
c cur_typ;
v_table_exists VARCHAR2(1);
type temp1_rec is record (col1 VARCHAR2(100), col2 VARCHAR2(100));
v_temp temp1_rec;
BEGIN
SELECT 'Y'
INTO v_table_exists
FROM all_tables
WHERE table_name = 'TEMP1';
--dynamic query with parameters
OPEN c FOR 'SELECT col1, col2 FROM temp1 WHERE :param1=:param2' USING 'PARAM1', 'PARAM1' ;
LOOP
FETCH c INTO v_temp;
EXIT WHEN c%NOTFOUND;
DBMS_OUTPUT.PUT_LINE(v_temp.col1);
END LOOP;
EXCEPTION
WHEN NO_DATA_FOUND THEN
NULL;
END;
/
CREATE TABLE TEMP1
(COL1 VARCHAR2(100),
col2 VARCHAR2(100));
INSERT INTO temp1
VALUES('123123123asdfasdfsfa', 'JHASDKLFJLASDFLAS');