Pl-Sql: Null Values in Comparisons and Conditional Statements

When working with nulls, you can avoid some common mistakes by keeping in mind the following rules:

Comparisons involving nulls always yield NULL

Applying the logical operator NOT to a null yields NULL

In conditional control statements, if the condition yields NULL, its associated sequence of statements is not executed

If the expression in a simple CASE statement or CASE expression yields NULL, it cannot be matched by using WHEN NULL. In this case, you would need to use the searched case syntax and test WHEN expression IS NULL.

In the example below, you might expect the sequence of statements to execute because x and y seem unequal. But, nulls are indeterminate. Whether or not x is equal to y is unknown. Therefore, the IF condition yields NULL and the sequence of statements is bypassed.

DECLARE
x NUMBER := 5;
y NUMBER := NULL;
BEGIN
IF x != y THEN -- yields NULL, not TRUE
dbms_output.put_line('x != y'); -- not executed
ELSIF x = y THEN -- also yields NULL
dbms_output.put_line('x = y');
ELSE
dbms_output.put_line('Can''t tell if x and y are equal or not...');
END IF;
END;
/
In the next example, you might expect the sequence of statements to execute because a and b seem equal. But, again, that is unknown, so the IF condition yields NULL and the sequence of statements is bypassed.

DECLARE
a NUMBER := NULL;
b NUMBER := NULL;
BEGIN
IF a = b THEN -- yields NULL, not TRUE
dbms_output.put_line('a = b'); -- not executed
ELSIF a != b THEN -- yields NULL, not TRUE
dbms_output.put_line('a != b'); -- not executed
ELSE
dbms_output.put_line('Can''t tell if two NULLs are equal');
END IF;
END;
/
NULLs and the NOT Operator
Recall that applying the logical operator NOT to a null yields NULL. Thus, the following two statements are not always equivalent:

IF x > y THEN | IF NOT x > y THEN
high := x; | high := y;
ELSE | ELSE
high := y; | high := x;
END IF; | END IF;

The sequence of statements in the ELSE clause is executed when the IF condition yields FALSE or NULL. If neither x nor y is null, both IF statements assign the same value to high. However, if either x or y is null, the first IF statement assigns the value of y to high, but the second IF statement assigns the value of x to high.


NULLs and Zero-Length Strings

PL/SQL treats any zero-length string like a null. This includes values returned by character functions and Boolean expressions. For example, the following statements assign nulls to the target variables:

DECLARE
null_string VARCHAR2(80) := TO_CHAR('');
address VARCHAR2(80);
zip_code VARCHAR2(80) := SUBSTR(address, 25, 0);
name VARCHAR2(80);
valid BOOLEAN := (name != '');
BEGIN
NULL;
END;
/
Use the IS NULL operator to test for null strings, as follows:

IF my_string IS NULL THEN ...

NULLs and the Concatenation Operator

The concatenation operator ignores null operands. For example, the expression

'apple' || NULL || NULL || 'sauce'

returns the following value:

'applesauce'

NULLs as Arguments to Built-In Functions

If a null argument is passed to a built-in function, a null is returned except in the following cases.

The function DECODE compares its first argument to one or more search expressions, which are paired with result expressions. Any search or result expression can be null. If a search is successful, the corresponding result is returned. In the following example, if the column rating is null, DECODE returns the value 1000:

DECLARE
the_manager VARCHAR2(40);
name employees.last_name%TYPE;
BEGIN
-- NULL is a valid argument to DECODE. In this case, manager_id is null
-- and the DECODE function returns 'nobody'.
SELECT DECODE(manager_id, NULL, 'nobody', 'somebody'), last_name
INTO the_manager, name FROM employees WHERE employee_id = 100;
dbms_output.put_line(name || ' is managed by ' || the_manager);
END;
/
The function NVL returns the value of its second argument if its first argument is null. In the following example, if the column specified in the query is null, the function returns the value -1 to signify a non-existent employee in the output:

DECLARE
the_manager employees.manager_id%TYPE;
name employees.last_name%TYPE;
BEGIN
-- NULL is a valid argument to NVL. In this case, manager_id is null
-- and the NVL function returns -1.
SELECT NVL(manager_id, -1), last_name
INTO the_manager, name FROM employees WHERE employee_id = 100;
dbms_output.put_line(name || ' is managed by employee #' || the_manager);
END;
/
The function REPLACE returns the value of its first argument if its second argument is null, whether the optional third argument is present or not. For example, the following call to REPLACE does not make any change to the value of OLD_STRING:

DECLARE
string_type VARCHAR2(60);
old_string string_type%TYPE := 'Apples and oranges';
my_string string_type%TYPE := 'more apples';
-- NULL is a valid argument to REPLACE, but does not match
-- anything so no replacement is done.
new_string string_type%TYPE := REPLACE(old_string, NULL, my_string);
BEGIN
dbms_output.put_line('Old string = ' || old_string);
dbms_output.put_line('New string = ' || new_string);
END;
/
If its third argument is null, REPLACE returns its first argument with every occurrence of its second argument removed. For example, the following call to REPLACE removes all the dashes from DASHED_STRING, instead of changing them to another character:

DECLARE
string_type VARCHAR2(60);
dashed string_type%TYPE := 'Gold-i-locks';
-- When the substitution text for REPLACE is NULL,
-- the text being replaced is deleted.
name string_type%TYPE := REPLACE(dashed, '-', NULL);
BEGIN
dbms_output.put_line('Dashed name = ' || dashed);
dbms_output.put_line('Dashes removed = ' || name);
END;
/
If its second and third arguments are null, REPLACE just returns its first argument.