CREATE TABLE example (
  col1 INT NOT NULL,          -- null checkbox unchecked, no default
  col2 INT NULL,              -- null checkbox checked, no default
  col3 INT NULL DEFAULT NULL, -- null checkbox checked, default null
  col4 INT NOT NULL DEFAULT 1 -- null checkbox unchecked, default value 1
);

INSERT INTO example(col1, col2, col3, col4) VALUES(1, 1, 1, 1); -- success
INSERT INTO example(col1, col2, col3, col4) VALUES(NULL, 1, 1, 1); -- fails, col1 cannot be null
INSERT INTO example(col1, col2, col3, col4) VALUES(1, NULL, 1, 1); -- success
INSERT INTO example(col1, col2, col3, col4) VALUES(1, 1, NULL, 1); -- success
INSERT INTO example(col1, col2, col3, col4) VALUES(1, 1, 1, NULL); -- fails, col4 cannot be null
INSERT INTO example(col2, col3, col4) VALUES(1, 1, 1); -- fails, col1 has no default value
INSERT INTO example(col1, col3, col4) VALUES(1, 1, 1); -- fails, col2 has no default value
INSERT INTO example(col1, col2, col4) VALUES(1, 1, 1); -- success, col3 is set to NULL
INSERT INTO example(col1, col2, col3) VALUES(1, 1, 1); -- success, col4 is set to 1