SQL Join生成重复的行

Question

我有以下PostgreSQL语句：

SELECT answer, answermark,answerid, answer.questionid, quizid 
FROM answer
LEFT JOIN  questions 
  ON  answermark = answer.questionid 
WHERE quizid = 1
ORDER BY answermark

然而，我得到的结果与我的3行重复 寻找。如何更改查询以消除重复项。

Answer 1

在select语句中使用distinct可以删除重复项

试试这个： -

SELECT distinct answer, answermark,answerid, answer.questionid, quizid FROM answer
LEFT JOIN  questions ON  answermark = answer.questionid WHERE quizid =1
ORDER BY answermark

Answer 2

您需要使用限定的列名来编写查询。每个列引用都应包含表名。这是一个好习惯;它会阻止您所看到的意外结果。

根据列名称，我认为您的查询是：

SELECT a.answer, a.answermark, a.answerid, a.questionid, q.quizid 
FROM answer LEFT JOIN 
     questions q
     ON  a.answermark = a.questionid 
---------^ I am guessing `answermark` comes from `answer`, not `questions`.
WHERE q.quizid = 1
ORDER BY a.answermark;

我不知道为什么名为answermark的列会与名为questionid的列进行比较，而不管表的来源如何。如果这是一个有效的比较，那么您的数据库的命名约定非常差。

我怀疑你想要这样的东西：

SELECT a.answer, a.answermark, a.answerid, a.questionid, q.quizid 
FROM answer LEFT JOIN 
     questions q
     ON q.questionid = a.questionid 
WHERE q.quizid = 1
ORDER BY a.answermark;