Commit fe93f48

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join优化

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Lines changed: 5 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -8,9 +8,13 @@ join从句:: inner \| full outer \| left outer \| right outer \| cross`
`8`	`8`
`9`	`9`
`10`	`10`
	`11`	`+********************************************************************************`
	`12`	`+left join 以左表为基础查询`
	`13`	`+忽然想起来有次面试问我如何用left join实现not in的功能 = =`
`11`	`14`
	`15`	`+select A.内容,B.内容 from A left join B ON a.key=b.key where b.key is null`
`12`	`16`
`13`		`-`
	`17`	`+not in 关键字不会使用索引所以用left join加上关键字为null查找只存在于A表不存在于B表的数据`
`14`	`18`
`15`	`19`
`16`	`20`

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