问题
Is there any difference in performance between the operator IS NULL and the function ISNULL()?
回答1:
Looking into the MySQL manual, they seem to be synonyms really.
MySQL manual on IS NULL
MySQL manual on ISNULL()
and even if they aren't, I would tend to trust the query optimizer to pick the best solution.
回答2:
This thread is similar, though not exactly on MySQL. According to the test shown there:
IS NULL is more efficient as it doesn't require a scan.
Seek is generally faster than a scan as it only includes qualifying records, while scan includes every row. It is explained in more detail here.
Another difference (though it's not performance) is their negation syntax:
IS NOT NULL /* using NOT operator */
! ISNULL() /* using exclamation mark */
来源：https://stackoverflow.com/questions/3530124/differences-between-is-null-and-isnull-in-mysql