sql - 反连接和反半连接有什么区别？

Question

来自 http://www.dbspecialists.com/files/presentations/semijoins.html

An “anti-join” between two tables returns rows from the first table where no matches are found in the second table. An anti-join is essentially the opposite of a semi-join: While a semi-join returns one copy of each row in the first table for which at least one match is found, an anti-join returns one copy of each row in the first table for which no match is found.

不是反半连接而不是反连接“返回第一个表中没有找到匹配项的每一行的一个副本”吗？

如果是，反加入的定义是什么？

请注意，我的问题是一般的 SQL 级别，例如大学教科书中关于数据库概念的级别，而不是特定于特定 SQL RDBMS 实现的问题

谢谢。

Answer 1

反半连接就是反连接。

The antijoin can also be defined as the complement of the semijoin, as follows:

R ▷ S = R − R ⋉ S

Given this, the antijoin is sometimes called the anti-semijoin, and the antijoin operator is sometimes written as semijoin symbol with a bar above it, instead of ▷.

参见 https://en.wikipedia.org/wiki/Relational_algebra#Antijoin_.28.E2.96.B7.29