If we change a NULL for that value, use the following functions (ISNULL, IFNULL, NVL, COLACESCE) according to the system database.
For our examples, we want if the value is NULL is changed by the value 0
Example of SQL SERVER using ISNULL:
Example for using ORACLE NVL:
Example for MySQL, there are 2 equivalent functions (IFNULL, COALESCE):