usergrade表
2.找到里面username是null的行
SELECT * FROM usergrade WHERE ISNULL(USERNAME)
SELECT * FROM usergrade WHERE USERNAME IS NULL
3.找到里面username不是null的行
SELECT * FROM usergrade WHERE USERNAME IS NOT NULL
4.列出所有的行，并且将username是null的字段替换为lxy
SELECT IFNULL(USERNAME,'lxy'),GRADE FROM usergrade
