Where and how to use NULLIF(X,Y) function of SQLITE? - sqlite

Question

I know that NULLIF(X,Y) function of SQLITE work equivalent to:
CASE
WHEN
X = Y
THEN
NULL
ELSE
X
END
and IFNULL(X,Y) function work equivalent to:
CASE
WHEN
X IS NULL
THEN
Y
ELSE
X
END
IFNULL(X,Y) function of SQLITE is used for replacing the NULL values of X to the Y but I can't understand the use of NULLIF(X,Y) function of SQLITE.
Please explain with examples, so it is more useful.

Answer 1

The IFNULL function is used when the database contains NULL values, but you want to handle those values as something else; for example:
SELECT Name, IFNULL(Age, 'unknown') AS Age FROM People
The NULLIF function is used when the database contains special values that are not NULL, but that you want to handle as NULL.
This is useful especially for aggregate functions. For example, to get the number of employees that get bonuses, use:
SELECT COUNT(NULLIF(Bonus, 0)) FROM Employees
This is the same as:
SELECT COUNT(*) FROM Employees WHERE Bonus != 0
In practice, NULLIF is not used as often as IFNULL.

Answer 2

I use NULLIF() when UPDATing or INSERTing rows containing NULLable fieds.
Basically with PHP :
$value1 = 'foo';
$value2 = '';
$sql = 'INSERT INTO table(field1, field2) '
. "VALUES(NULLIF('$value1', ''), NULLIF('$value2', ''))";
// => INSERT INTO table(field1, field2) VALUES(NULLIF('foo', ''), NULLIF('', ''))
// => INSERT INTO table(field1, field2) VALUES('foo', NULL)
It saves me doing things like :
$value1forSQL = ($value1 === '' || $value1 === NULL) ? 'NULL' : "'$value1'";
...
$sql = ...
. "VALUES($value1forSQL, ...)";