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

问题描述

在尝试理解半联接的概念以及它与常规联接有何不同时，我遇到了麻烦.我已经尝试过一些文章，但对解释不满意，有人可以帮助我理解它吗?

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.让我们使用左外部联接选择成绩得分的学生:

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

现在与左半联接相同:

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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