remove not null in sql code example
Example 1: sql not null
SELECT column_name FROM table_name
WHERE column_name IS NOT NULL;
Example 2: mysql on delete set null
FOREIGN KEY (child_col1, child_col2, ... child_col_n)
REFERENCES parent_table (parent_col1, parent_col2, ... parent_col_n)
ON DELETE SET NULL
[ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]