创建数据库、使用数据库、查看表格：
MariaDB [mydb]> create database mydb; <==创建数据库
MariaDB [mydb]> use mydb; <==使用数据库
MariaDB [mydb]> show tables; <==查看表格
+----------------+
| Tables_in_mydb |
+----------------+
| stu |
| students |
| tbl1 |
+----------------+
3 rows in set (0.00 sec)
创建表格：
MariaDB [mydb]> create table students (stuid INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,name VARCHAR(200),age TINYINT UNSIGNED,gender ENUM('F','M'),major VARCHAR(200));
#创建表格students
插入数据到表格：
MariaDB [mydb]> INSERT INTO students (name,age,gender,major) value('jia baoyu',17,'M','pixie jianfa'),('li chong',37,'M','xialong shibazhang'),('ximen qing',31,'M','kuihua baodian'),('li mochou',27,'F','wuxiang shengong');
#插入数据到表格students
[root@centos7b ~]#for i in {5..1000};do mysql -p123456 -e "INSERT INTO mydb.students VALUES ('$i','stu$i',$[$RANDOM%100+1],'M','major$i')";done
#批量插入数据到mydb数据库的students表格
查看数据：
MariaDB [mydb]> select name,age from students where age >= 15;
#显示表格students内，age>=15的行
创建索引：
MariaDB [mydb]> create index name ON students(name);
#根据字段‘name’，给表格students创建索引
MariaDB [mydb]> show indexes from students;
#查看表格 students的索引信息
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students | 0 | PRIMARY | 1 | stuid | A | 2204 | NULL | NULL | | BTREE | | |
| students | 1 | name | 1 | name | A | 220 | NULL | NULL | YES | BTREE | | |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
使用索引的好处：
MariaDB [mydb]> explain select * from students where name = 'stu999' \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: ref
possible_keys: name
key: name
key_len: 203
ref: const
rows: 1 <==使用索引“name”过滤，找出结果只扫描了一行
Extra: Using index condition
1 row in set (0.00 sec)
MariaDB [mydb]> explain select * from students where major = 'major999' \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 2204 <==不使用索引，找出结果扫描了2204行
Extra: Using where
1 row in set (0.00 sec)
右侧通配，索引有效：
MariaDB [mydb]> explain select * from students where name like 'stu100%' \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: range
possible_keys: name
key: name
key_len: 203
ref: NULL
rows: 11
Extra: Using index condition
1 row in set (0.00 sec)
左侧通配，索引失效：
MariaDB [mydb]> explain select * from students where name like '%tu100%' \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 2204
Extra: Using where
1 row in set (0.00 sec)
创建多列索引
MariaDB [mydb]> create index name_and_age ON students(name,age);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [mydb]> show index from students;
+----------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students | 0 | PRIMARY | 1 | stuid | A | 2054 | NULL | NULL | | BTREE | | |
| students | 1 | name | 1 | name | A | 2054 | NULL | NULL | YES | BTREE | | |
| students | 1 | name_and_age | 1 | name | A | 186 | NULL | NULL | YES | BTREE | | |
| students | 1 | name_and_age | 2 | age | A | 186 | NULL | NULL | YES | BTREE | | |
+----------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.00 sec)
MariaDB [mydb]> explain select * from students where name like 'stu100%' \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: range
possible_keys: name,name_and_age <==过滤name，可用的索引有两个，因此我们可以把‘name’这个索引删除了，只留下‘name_and_age ’
key: name
key_len: 203
ref: NULL
rows: 11
Extra: Using index condition
1 row in set (0.00 sec)
MariaDB [mydb]> explain select * from students where age >= 80 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: ALL
possible_keys: NULL <==只过滤age，没有可用索引。‘name_and_age ’不能跳过name，只索引age。
key: NULL
key_len: NULL
ref: NULL
rows: 1611
Extra: Using where
1 row in set (0.00 sec)
MariaDB [mydb]> drop index name on students;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
MariaDB [mydb]> show index from students;
+----------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students | 0 | PRIMARY | 1 | stuid | A | 1611 | NULL | NULL | | BTREE | | |
| students | 1 | name_and_age | 1 | name | A | 1611 | NULL | NULL | YES | BTREE | | |
| students | 1 | name_and_age | 2 | age | A | 1611 | NULL | NULL | YES | BTREE | | |
+----------+------------+--------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
MariaDB [mydb]> explain select * from students where name like 'stu100%' and age >= 80 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: range
possible_keys: name_and_age <==同时过滤name和age，可以用索引
key: name_and_age <==使用索引name_and_age
key_len: 205
ref: NULL
rows: 11
Extra: Using index condition
1 row in set (0.00 sec)
MariaDB [mydb]> explain select * from students where name like 'stu%' and age >= 80 \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: students
type: ALL
possible_keys: name_and_age <==可以用索引
key: NULL <==不用索引（此处因为stu%匹配的行太多，即使有索引‘name_and_age ’ ，系统也并没有用到索引它）
key_len: NULL
ref: NULL
rows: 1611
Extra: Using where
1 row in set (0.00 sec)