MySQL不插入重复数据与删除重复数据

今天写个python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'
)

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

小记于此 备查