When you perform divisions, you need to make sure that the divisor, the number to the right of the division operator, isn’t zero. If it is, you will end up with a division-by-zero error message:

Msg 8134, Level 16, State 1, Line 1
Divide by zero error encountered.

Dividing by NULL, however, doesn’t. So to avoid division-by-zero errors, simply use NULLIF() to replace zeros in the divisor with a NULL value.

How the NULLIF() function works

The NULLIF() function is pretty much the opposite of the ISNULL() function. ISNULL(a, b) returns a, except if a is NULL. If a is NULL, it returns b.

NULLIF(a, b), however, returns NULL if a is equal to b.

The T-SQL

Here’s the basic set-up, a temp table with some random values:

CREATE TABLE #test (
    a    numeric(10, 2) NOT NULL,
    b    numeric(10, 2) NOT NULL
);

INSERT INTO #test (a, b)
VALUES (7, 2), (3, 0), (5, 7), (9, 2);

The following query will return an error:

SELECT a, b, a/b
FROM #test;

But if you use NULLIF() in the divisor, the result of the division will instead be NULL, which is pretty close to the “undefined” result of a zero division.

SELECT a, b, a/NULLIF(b, 0.0)
FROM #test;