1 system 好像数据库只有一条才会出现

2 const 主键or 唯一索引查询

主键查询
explain

select * from student where id  = 1

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唯一索引查询
UNIQUE KEY `name-key` (`name`) USING BTREE

explain
select * from student where name  = '1'

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3 eq-ref

4 ref 普通索引

KEY `name-key` (`name`) USING BTREE

explain
select * from student where name  =  '我是一个美国人'

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5 ref-or-null

6 index-merge

7 index_subquery

8 range

普通索引或者唯一索引  like查询
KEY `name-key` (`name`) USING BTREE

explain
select * from student where name  like  '我是一个美国人%'

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9 index

10 all 全表扫描

explain
select * from student 

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