DELETE FROM user WHERE username IS NULL;
To Delete the Rows with the Null values if present in any specific Columns.SET foreign_key_checks = 0;
UPDATE languages SET id='xyz' WHERE id='abc';
UPDATE categories_languages SET language_id='xyz' WHERE language_id='abc';
SET foreign_key_checks = 1;
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 } ]