SQL 之 查詢操作重復記錄

　　有時，我們的數據表中會存在一些冗余數據，這就要求我們查詢並操作這些冗余數據。

一、查詢表中重復記錄

　　例如，查找重復記錄是根據單個字段（peopleId）來判斷 

SELECT * FROM Tpeople 
  WHERE peopleId IN ( SELECT peopleId FROM Tpeople GROUP BY peopleId HAVING COUNT(peopleId) > 1)

二、刪除表中多余的重復記錄

　　例如，重復記錄是根據單個字段（peopleId）來判斷，只保留最先增加的記錄，下面是保留ID最小的記錄

DELETE FROM Tpeople 
  WHERE peopleName IN ( SELECT peopleName FROM Tpeople GROUP BY peopleName HAVING COUNT(peopleName)>1) 
    AND peopleId NOT IN ( SELECT MIN(peopleId) FROM Tpeople GROUP BY peopleName HAVING COUNT(peopleName)>1) 

三、查找表中多余的重復記錄（多個字段）
　　A，DB2中可以如下查詢

SELECT * FROM vitae TA
 WHERE (TA.peopleId, TA.seq) IN ( SELECT peopleId,seq FROM vitae TB GROUP BY peopleId,seq HAVING COUNT(*) > 1)

　　B，SQLServer如下查詢

SELECT * FROM vitae TA
 WHERE EXISTS ( SELECT * FROM vitae TB WHERE TB.peopleId=TA.peopleId AND TB.seq =TA.seq GROUP BY peopleId,seq HAVING COUNT(1) > 1)

四、刪除表中多余的重復記錄（多個字段），只留有最先插入的記錄

　　A，DB2中可以如下刪除

DELETE FROM vitae TA
  WHERE (TA.peopleId,TA.seq) IN (SELECT peopleId,seq FROM vitae TB GROUP BY peopleId,seq HAVING COUNT(1) > 1) 
    AND rowid NOT IN ( SELECT MIN(rowid) FROM vitae TC GROUP BY peopleId,seq HAVING COUNT(1)>1)

　　B，SQLServer中如下刪除

DELETE FROM vitae TA
  WHERE EXISTS ( SELECT * FROM vitae TB WHERE TB.peopleId=TA.peopleId AND TB.seq =TA.seq GROUP BY peopleId,seq HAVING COUNT(1) > 1)
    AND rowid NOT IN ( SELECT MIN(rowid) FROM vitae TC GROUP BY peopleId,seq HAVING COUNT(1)>1)