Adds SQL function isnull(column)

[killme2008]

What type of enhancement is this?

API improvement, User experience

What does the enhancement do?

There is a function isnull in MySQL to test if a column is null:

SELECT ISNULL(NULL);

https://www.w3schools.com/sql/func_mysql_isnull.asp

We need it for Navicat compatibility.

Implementation challenges

If someone is interested in how to implement an SQL function, please refer to #3147

[KKould]

let me try this