in where condition，have null value，then the sql will be
UPDATE user SET age = 18 WHERE name = NULL;
but name = NULL will result 0 rows. however there is one row with null name.
change the sql like that:
UPDATE user SET age = 18 WHERE name IS NULL;
then the row with NULL name will be updated.
in where condition，have null value，then the sql will be UPDATE user SET age = 18 WHERE name = NULL; but name = NULL will result 0 rows. however there is one row with null name. change the sql like that: UPDATE user SET age = 18 WHERE name IS NULL; then the row with NULL name will be updated.