MySQL可以在单独一列上添加B-Tree索引，也可以在多列数据上添加B-Tree索引，多列的数据按照添加索引声明的顺序组合起来，存储在B-Tree的页中。假设有如下数据表： sql
CREATE TABLE People (
last_name varchar(50) not null,
first_name varchar(50) not null,
birthday date not null,
gender enum('m','f') not null
key(last_name, first_name, birthday)
);
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mysql>SELECT * FROM tb1 WHERE b BETWEEN 2 AND 3;
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mysql> EXPLAIN SELECT actor_id, MAX(film_id)
-> FROM sakila.film.film_actor
-> GROUP BY actor_id;
********************************************* 1. row ***********************************
id: 1
select_type: SIMPLE
table: film_actor
type: range
possible_keys: NULL
key: PRIMARY
key_len: 2
ref: NULL
rows: 396
Extra: Using index for group-by
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mysql> EXPLAIN SELECT store_id, film_id FROM sakila.inventory
*********************************1.row***************************************
id:1
select_type:SIMPLE
table:inventory
type:index
possible_keys:NULL
key:idx_store_id_film_id
key_len:3
ref:NULL
rows:4673
Extra:Using Index
图1 B-Tree索引的底层结构示意图
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