SELECT statement (urgent!)

Question

Could you please help me with the following statement?
The FROM clause is incorrect but i don't know what i should write.
Thank you in advance.
Maria.
DECLARE
A NUMBER;
CURSOR C1 IS
SELECT TABLE_NAME, COLUMN_NAME
FROM USER_TAB_COLUMNS
WHERE NULLABLE='N' AND TABLE_NAME LIKE 'P_%';
BEGIN
FOR i IN C1
LOOP
SELECT COUNT(*) INTO A
FROM i.TABLE_NAME
WHERE i.COLUMN_NAME IS NOT NULL;
END LOOP;
END;
/

Answer 1

FOR-LOOP
Whereas the number of iterations through a WHILE loop is unknown until the loop completes, the number of iterations through a FOR loop is known before the loop is entered. FOR loops iterate over a specified range of integers. The range is part of an iteration scheme, which is enclosed by the keywords FOR and LOOP. A double dot (..) serves as the range operator. The syntax follows:
FOR counter IN [REVERSE] lower_bound..higher_bound LOOP
sequence_of_statements
END LOOP;
The range is evaluated when the FOR loop is first entered and is never re-evaluated.
As the next example shows, the sequence of statements is executed once for each integer in the range. After each iteration, the loop counter is incremented.
FOR i IN 1..3 LOOP -- assign the values 1,2,3 to i
sequence_of_statements -- executes three times
END LOOP;
The following example shows that if the lower bound equals the higher bound, the sequence of statements is executed once:
FOR i IN 3..3 LOOP -- assign the value 3 to i
sequence_of_statements -- executes one time
END LOOP;
By default, iteration proceeds upward from the lower bound to the higher bound. However, as the example below shows, if you use the keyword REVERSE, iteration proceeds downward from the higher bound to the lower bound. After each iteration, the loop counter is decremented. Nevertheless, you write the range bounds in ascending (not descending) order.
FOR i IN REVERSE 1..3 LOOP -- assign the values 3,2,1 to i
sequence_of_statements -- executes three times
END LOOP;
Inside a FOR loop, the loop counter can be referenced like a constant but cannot be assigned values, as the following example shows:
FOR ctr IN 1..10 LOOP
IF NOT finished THEN
INSERT INTO ... VALUES (ctr, ...); -- legal
factor := ctr * 2; -- legal
ELSE
ctr := 10; -- not allowed
END IF;
END LOOP;
Iteration Schemes
The bounds of a loop range can be literals, variables, or expressions but must evaluate to numbers. Otherwise, PL/SQL raises the predefined exception VALUE_ERROR. The lower bound need not be 1, as the examples below show. However, the loop counter increment (or decrement) must be 1.
j IN -5..5
k IN REVERSE first..last
step IN 0..TRUNC(high/low) * 2
Internally, PL/SQL assigns the values of the bounds to temporary PLS_INTEGER variables, and, if necessary, rounds the values to the nearest integer. The magnitude range of a PLS_INTEGER is -2**31 .. 2**31. So, if a bound evaluates to a number outside that range, you get a numeric overflow error when PL/SQL attempts the assignment, as the following example shows:
DECLARE
hi NUMBER := 2**32;
BEGIN
FOR j IN 1..hi LOOP -- causes a 'numeric overflow' error
END LOOP;
END;
Some languages provide a STEP clause, which lets you specify a different increment (5 instead of 1 for example). PL/SQL has no such structure, but you can easily build one. Inside the FOR loop, simply multiply each reference to the loop counter by the new increment. In the following example, you assign today's date to elements 5, 10, and 15 of an index-by table:
DECLARE
TYPE DateList IS TABLE OF DATE INDEX BY BINARY_INTEGER;
dates DateList;
k CONSTANT INTEGER := 5; -- set new increment
BEGIN
FOR j IN 1..3 LOOP
dates(j*k) := SYSDATE; -- multiply loop counter by increment
END LOOP;
END;
Dynamic Ranges
PL/SQL lets you determine the loop range dynamically at run time, as the following example shows:
SELECT COUNT(empno) INTO emp_count FROM emp;
FOR i IN 1..emp_count LOOP
END LOOP;
The value of emp_count is unknown at compile time; the SELECT statement returns the value at run time.
What happens if the lower bound of a loop range evaluates to a larger integer than the upper bound? As the next example shows, the sequence of statements within the loop is not executed and control passes to the next statement:
-- limit becomes 1
FOR i IN 2..limit LOOP
sequence_of_statements -- executes zero times
END LOOP;
-- control passes here
Scope Rules
The loop counter is defined only within the loop. You cannot reference it outside the loop. After the loop is exited, the loop counter is undefined, as the following example shows:
FOR ctr IN 1..10 LOOP
END LOOP;
sum := ctr - 1; -- not allowed
You need not explicitly declare the loop counter because it is implicitly declared as a local variable of type INTEGER. The next example shows that the local declaration hides any global declaration:
DECLARE
ctr INTEGER;
BEGIN
FOR ctr IN 1..25 LOOP
IF ctr > 10 THEN ... -- refers to loop counter
END LOOP;
END;
To reference the global variable in this example, you must use a label and dot notation, as follows:
<<main>>
DECLARE
ctr INTEGER;
BEGIN
FOR ctr IN 1..25 LOOP
IF main.ctr > 10 THEN -- refers to global variable
END IF;
END LOOP;
END main;
The same scope rules apply to nested FOR loops. Consider the example below. Both loop counters have the same name. So, to reference the outer loop counter from the inner loop, you must use a label and dot notation, as follows:
<<outer>>
FOR step IN 1..25 LOOP
FOR step IN 1..10 LOOP
IF outer.step > 15 THEN ...
END LOOP;
END LOOP outer;
Using the EXIT Statement
The EXIT statement lets a FOR loop complete prematurely. For example, the following loop normally executes ten times, but as soon as the FETCH statement fails to return a row, the loop completes no matter how many times it has executed:
FOR j IN 1..10 LOOP
FETCH c1 INTO emp_rec;
EXIT WHEN c1%NOTFOUND;
END LOOP;
Suppose you must exit from a nested FOR loop prematurely. You can complete not only the current loop, but any enclosing loop. Simply label the enclosing loop that you want to complete. Then, use the label in an EXIT statement to specify which FOR loop to exit, as follows:
<<outer>>
FOR i IN 1..5 LOOP
FOR j IN 1..10 LOOP
FETCH c1 INTO emp_rec;
EXIT outer WHEN c1%NOTFOUND; -- exit both FOR loops
END LOOP;
END LOOP outer;
-- control passes here
Joel P�rez