2021.7.12 internal and external connection of notes

Link query

Cartesian product phenomenon and limiting conditions

Therefore, we need to add restrictions

You can also rename , Increase search efficiency （SQL92 grammar ）（ Even if it turns out to be 14 strip , Also need to find all play 56 strip ）

SQL99 grammar （ The same result ）：

SQL92 and SQL99 And equivalent connection

join There can be a inner But you can omit , But with inner Better readability , Indicates internal connection

ps： Equivalent connection means that the condition is an equal quantity relationship （ As in the case e.deptno = d.deptno）

Non equivalent connection

The condition is non equivalent （ there inner It can also be omitted ）

Self join

Find the leader of each employee （mgr = empno）

King There is no leader, so the query result is 13 strip （ in total 14 strip ）

External connection

stay join The front is left（right） To the left （ Right ） Connect

Three meter connection