MySQL不插入重复数据与删除重复数据

今天写个小程序需要用到mysql,需求就是要求插入数据，但该数据不能已存在于表中，否则就不插入。

比如我需要插入question="aaa",correctAnswer="bbb"的数据，代码如下：

INSERT  INTO  questions
(question,correctAnswer)
SELECT  'aaa' , 'bbb'  FROM  dual
where  not  exists(
SELECT  *  FROM  questions
where  question= 'aaa'
and  correctAnswer= 'bbb'
)

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其中"dual"为虚表，用于不提供实际表名查询数据。

当然，如果已经插入了若干条重复数据，则可以这么删除:

DELETE  FROM  a USING questions  AS  a,
(
SELECT  *
FROM  questions
GROUP  BY  question, correctAnswer
HAVING  COUNT ( * ) >1
)  AS  b
WHERE  a.question = b.question
AND  a.correctAnswer = b.correctAnswer
AND  a.questionId > b.questionId

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