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 


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