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Merge branch 'master' of github.com:Snailclimb/JavaGuide

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`@@ -179,13 +179,6 @@ ALTER TABLE table ADD INDEX index_name (num,name,age)`
`179`	`179`
`180`	`180`	`那么当查询的条件有为:num / (num AND name) / (num AND name AND age)时,索引才生效。所以在创建联合索引时,尽量把查询最频繁的那个字段作为最左(第一个)字段。查询的时候也尽量以这个字段为第一条件。`
`181`	`181`
`182`		`-> 但可能由于版本原因(我的 mysql 版本为 8.0.x),我创建的联合索引,相当于在联合索引的每个字段上都创建了相同的索引:`
`183`		`-`
`184`		`-![联合索引(多列索引)](<../../media/pictures/database/联合索引(多列索引).png>)`
`185`		`-`
`186`		`-无论是否符合最左前缀原则,每个字段的索引都生效:`
`187`		`-`
`188`		`-![联合索引生效](../../media/pictures/database/联合索引之查询条件生效.png)`
`189`	`182`
`190`	`183`	`## 索引创建注意点`
`191`	`184`

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