How to conditionally handle division by zero with MySQL

Question

In MySQL, this query might throw a division by zero error:

SELECT ROUND(noOfBoys / noOfGirls) AS ration
FROM student;

If noOfGirls

Answer 1

GREATEST(x, 1)

Can be used since the number of girls must be positive:

SELECT ROUND(noOfBoys / GREATEST(noOfGirls, 1)) AS ration FROM student;

But it is not very portable across SQL implementations.

Documentation: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_greatest

Answer 2

Well if you want to set noOfGirls to 1 if it's 0, then the following should work:

SELECT ROUND(noOfBoys / if(noOfGirls, noOfGirls, 1)) as ration FROM student;

Answer 3

check if the denominator is zero and the nominator is not NULL - if yes then use 'NaN'.

Answer 4

Division by NULL actually works in MySQL server and returns NULL. So you can do:

SELECT ROUND(noOfBoys / NULLIF(noOfGirls, 0)) AS ration FROM student;

I think NULL ratio is the most accurate for this case. If you need the ratio to be equal to numOfBoys then you can use:

SELECT COALESCE(ROUND(noOfBoys / NULLIF(noOfGirls, 0)), noOfBoys) AS ration FROM student;

Answer 5

You can use this (over-expressive) way:

select IF(noOfGirls=0, NULL, round(noOfBoys/noOfGirls)) as ration from student;

Which will put out NULL if there are no girls, which is effectively what 0/0 should be in SQL semantics.

MySQL will anyway give NULL if you try to do 0/0, as the SQL meaning fo NULL is "no data", or in this case "I don't know what this value can be".

Answer 6

select
    case student.noOfGirls
           when 0 then 1
           else  round(noOfBoys/noOfGirls)
    end as ration
from
    `student`