1.select count() from mysql.user;//查找mysql库的user表的行数
mysql> select count() from mysql.user;//count()在myisam会很快、innoDB引擎就会很慢（不会去统计行数，很耗时），所有的操作尽量减少
+----------+
| count(*) |
+----------+
|8 |
+----------+

2.select from mysql.db;
mysql> select from mysql.db\G; //查找mysql库的db表所有内容
1. row
Host: %
Db: test
User:
Select_priv: Y
Insert_priv: Y
Update_priv: Y

3.select db from mysql.db;
select db from mysql.db; //查找mysql库的db表为db的字段
+---------+
| db|
+---------+
| test|
| test_% |
+---------+

4.select db,user from mysql.db;
select db,user from mysql.db; //可以查询两个字段
+---------+--------+
| db| user |
+---------+--------+
| test||
| test_% ||
| db1 | user2|
| discuz| discuz |
| db1 | user1|
| db1 | user3|
| discuz| discuz |
+---------+--------+
7 rows in set (0.00 sec)

5.select from mysql.db where host like '192.168.%'; //like模糊匹配
mysql> select from mysql.db where host like '192.168.%'\G;
2. row
Host: 192.168.211.%
Db: discuz
User: discuz
Select_priv: Y
Insert_priv: Y
Update_priv: Y
Delete_priv: Y

6.insert into db1.t1 values (1, 'abc'); //插入
mysql> insert db1.t1 values(1,'abc');
Query OK, 1 row affected (0.11 sec)
mysql> select * from db1.t1;
+------+------+
| id | name |
+------+------+
|1 | abc|
+------+------+
1 row in set (0.01 sec)

desc db1.t1; //查看表的结构
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id| int(4) | YES| | NULL| |
| name| char(40) | YES| | NULL| |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into db1.t1 values (1,'abc');//插入语句.也可以做一些限制，一样的值，不一样的id，一样的数字就报冲突
Query OK, 1 row affected (0.21 sec)
select * from db1.t1;
+------+------+
| id | name |
+------+------+
|1 | abc|
+------+------+

7.update db1.t1 set name='aaa' where id=1;//把db1库的t1表id为1的名字改为aaa
mysql> select from db1.t1;
+------+------+
| id | name |
+------+------+
|1 | abc|
|1 | lsx|
|3 | lshx |
+------+------+
3 rows in set (0.01 sec)
mysql> update db1.t1 set name='lsx01' where id=1;
Query OK, 2 rows affected (0.55 sec)
Rows matched: 2Changed: 2Warnings: 0
mysql> select from db1.t1;
+------+-------+
| id | name|
+------+-------+
|1 | lsx01 |
|1 | lsx01 |

8.delete from db1.t1 where id=6;//删除db库的t1表id为6的行
mysql> select from db1.t1;
+------+------+
| id | name |
+------+------+
|1 | abc|
|111 | lsx|
|111 | lshx |
+------+------+
3 rows in set (0.00 sec)
mysql> delete from db1.t1 where id=111;
Query OK, 2 rows affected (0.28 sec)
mysql> select from db1.t1;
+------+------+
| id | name |
+------+------+
|1 | abc|
+------+------+
1 row in set (0.00 sec)

9.truncate table db1.t1;//清空t1表内容。表结构留着
10.drop table db1.t1;//库结构、内容全部清空。
11.drop database db1.t1;//删除t1表的内容以及结构
12.drop database db1；//删除数据库