Question

我对SQL很新，所以我不确定我要做的事情是否被称为连接。

我有4个表，使用此架构：

CREATE TABLE survey (
    id           serial    PRIMARY KEY,
    title        text,
    password     text
);

CREATE TABLE question (
    id           serial    PRIMARY KEY,
    surveyId     integer   REFERENCES survey(id),
    value        text
);

CREATE TABLE answer (
    id           serial    PRIMARY KEY,
    questionId   integer   REFERENCES question(id) ,
    value text
);

CREATE TABLE vote (
    id           serial,
    questionId   integer   REFERENCES question(id),
    answerId     integer   REFERENCES answer(id)
);

鉴于具体的survey.id（surveyId），我需要：

查找question.surveyId = surveyId
对于（1）中找到的每个问题行，找到answer.questionId = question.id
对于（2）中找到的每个答案，找到vote.answerId = answer.id的所有投票行。
返回每个唯一answerId的投票行数。

所以，如果我有以下测试数据：

question
=============
id | surveyId
1  | 3
2  | 3
3  | 3
4  | 5
5  | 6


answer
===============
id | questionId
1  | 1
2  | 1
3  | 2
4  | 3


vote
==========================
id | questionId | answerId
1  | 1          | 1
2  | 1          | 2
3  | 1          | 1
4  | 2          | 3
5  | 4          | 22

如果初始surveyId为3，那么我希望此查询的结果为：

answerId | count
================
1        | 2
2        | 1
3        | 1

有没有办法以单个SQL查询的方式执行此操作？

Answer 1

以下是您需要的查询：

SELECT v.answerId, COUNT(*) 
FROM survey AS s 
     INNER JOIN question AS q ON s.id = q.surveyId
     INNER JOIN answer AS a ON q.id = a.questionId
     INNER JOIN vote AS v ON a.id = v.answerId
WHERE  s.id = 3 -- This's your specific value of surveyId
GROUP BY v.answerId 
ORDER BY v.answerId

但可以优化此查询。根据表的关系，您可以放弃单个连接：

SELECT v.answerId, COUNT(*) 
FROM survey AS s 
     INNER JOIN question AS q ON s.id = q.surveyId AND s.id = 3
     INNER JOIN vote AS v ON q.id = v.questionId
GROUP BY v.answerId 
ORDER BY v.answerId

请注意，您无法创建这样的测试数据集。因为表vote中的answerId值违反了此表中的外键约束answerId。表22中没有answer。

Answer 2

SELECT 
  a.answerId
  , vote = COUNT(v.vote) 
FROM survey s
  INNER JOIN question q
    ON s.surveyid = q.surveyId
  INNER JOIN answer a
    ON q.questionId = a.questionID
  INNER JOIN vote v
    ON a.answerId = v.answerId
WHERE s.surveyID = @yourValue
GROUP BY a.answerId

我认为你在寻找什么。

SQL查询加入3个表

2 个答案: