什么是数据库中的半连接? [英] What is semi-join in database?

问题描述

我在尝试理解半连接的概念以及它与传统连接的不同之处时遇到了问题.我已经尝试了一些文章但对解释不满意，有人可以帮助我理解吗?

I am having trouble while trying to understand the concept of semi-join and how it is different from conventional join. I have tried some article already but not satisfied with the explanation, could someone please help me to understand it?

推荐答案

简单示例.让我们使用左外连接选择有成绩的学生:

Simple example. Let's select students with grades using left outer join:

SELECT DISTINCT s.id
FROM  students s
      LEFT JOIN grades g ON g.student_id = s.id
WHERE g.student_id IS NOT NULL

现在与左半连接相同:

SELECT s.id
FROM  students s
WHERE EXISTS (SELECT 1 FROM grades g
              WHERE g.student_id = s.id)

后者效率更高.

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