字中复制mysql> select lower(DDD); +--------------+| lower(DDD) |+--------------+|    ddd |+--------------+mysql> select upper(ddd);   +--------------+| upper(ddd) |+--------------+| DDD |+--------------+mysql> select lcase(DDD);   +--------------+| lcase(DDD) |+--------------+| ddd |+--------------+mysql> select ucase(ddd); +--------------+| ucase(ddd) |+--------------+| DDD |+--------------+  1.2.3.4.5.6.