13.5 常用sql语句 - 码农教程

常用sql语句目录概要

select count(*) from mysql.user;
select * from mysql.db;
select db from mysql.db;
select db,user from mysql.db;
select * from mysql.db where host like '192.168.%';
insert into db1.t1 values (1, 'abc');
update db1.t1 set name='aaa' where id=1;
truncate table db1.t1;
drop table db1.t1;
drop database db1;

常用sql语句

增删改查，就是mysql和其他关系型数据库常用的select语句操作命令

查询语句

首先登录root下的mysql

[root@hf-01 ~]#  mysql -uroot -p'hanfeng'
Warning: Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 18
Server version: 5.6.35 MySQL Community Server (GPL)

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or 'h' for help. Type 'c' to clear the current input statement.

mysql>

使用db1库

mysql> use db1;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql>

查看当前库的所有表show tables;

mysql> show tables;
+---------------+
| Tables_in_db1 |
+---------------+
| t1            |
+---------------+
1 row in set (0.00 sec)

mysql>

查看表的行数 select count(*) from mysql.user;

库和表中间有个分割符，就是用点 . 分割

mysql> select count(*) from mysql.user;
+----------+
| count(*) |
+----------+
|       12 |
+----------+
1 row in set (0.01 sec)

mysql>

就是说user表有12行内容

查看所有的内容 select * from mysql.db;（这样看起来会很乱） ——>可以在后面加上G，如select * from mysql.dbG;

这里的 * 表示查看所有内容

mysql> select * from mysql.db/G;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '/G' at line 1
mysql> select * from mysql.dbG;
*************************** 1. row ***************************
                 Host: %
                   Db: test
                 User: 
          Select_priv: Y
          Insert_priv: Y
          Update_priv: Y
          Delete_priv: Y
          Create_priv: Y
            Drop_priv: Y
           Grant_priv: N
      References_priv: Y
           Index_priv: Y
           Alter_priv: Y
Create_tmp_table_priv: Y
     Lock_tables_priv: Y
     Create_view_priv: Y
       Show_view_priv: Y
  Create_routine_priv: Y
   Alter_routine_priv: N
         Execute_priv: N
           Event_priv: Y
         Trigger_priv: Y
*************************** 2. row ***************************
 等等等，只截取了一部分

查看db库的所有内容 select db from mysql.db; 第一个db是字段

mysql> select db from mysql.db;
+---------+
| db      |
+---------+
| test    |
| test_% |
| db1     |
| db1     |
+---------+
4 rows in set (0.01 sec)

mysql>

查db字段和user字段 select db,user from mysql.db;

mysql> select db,user from mysql.db;
+---------+-------+
| db      | user  |
+---------+-------+
| test    |       |
| test_% |       |
| db1     | user2 |
| db1     | user2 |
+---------+-------+
4 rows in set (0.00 sec)

mysql>

模糊查询 select * from mysql.db where host like '192.168.%';

like 就是模糊匹配

mysql> select * from mysql.db where host like '192.168.%'G;
*************************** 1. row ***************************
                 Host: 192.168.133.1
                   Db: db1
                 User: user2
          Select_priv: Y
          Insert_priv: Y
          Update_priv: Y
          Delete_priv: N
          Create_priv: N
            Drop_priv: N
           Grant_priv: N
      References_priv: N
           Index_priv: N
           Alter_priv: N
Create_tmp_table_priv: N
     Lock_tables_priv: N
     Create_view_priv: N
       Show_view_priv: N
  Create_routine_priv: N
   Alter_routine_priv: N
         Execute_priv: N
           Event_priv: N
         Trigger_priv: N
*************************** 2. row ***************************
                 Host: 192.168.133.2
                   Db: db1
                 User: user2
          Select_priv: Y
          Insert_priv: Y
          Update_priv: Y
          Delete_priv: N
          Create_priv: N
            Drop_priv: N
           Grant_priv: N
      References_priv: N
           Index_priv: N
           Alter_priv: N
Create_tmp_table_priv: N
     Lock_tables_priv: N
     Create_view_priv: N
       Show_view_priv: N
  Create_routine_priv: N
   Alter_routine_priv: N
         Execute_priv: N
           Event_priv: N
         Trigger_priv: N
2 rows in set (0.00 sec)

ERROR: 
No query specified

mysql>

插入语句

查看创建的表

mysql> desc db1.t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(4)   | YES  |     | NULL    |       |
| name  | char(40) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql>

查看db1.t1表的内容，会发现为空

mysql> select * from db1.t1;
Empty set (0.00 sec)

mysql>

插入数据到 insert into db1.t1 values (1, 'abc');

插入1, 'abc'到db1.t1表

mysql> insert into db1.t1 values (1, 'abc');
Query OK, 1 row affected (0.02 sec)

mysql>

再来查询db1.t1

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | abc  |
+------+------+
1 row in set (0.00 sec)

mysql>

这样就成功了插入了一条数据，在插入的时候 name 这个字段应该是是一个字符串，字符串需要加上一个单引号 ' ' ，数字可以不加单引号

mysql> insert into db1.t1 values (1, 234);
Query OK, 1 row affected (0.01 sec)

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | abc  |
|    1 | 234  |
+------+------+
2 rows in set (0.00 sec)

mysql>

5.这里没有做限制，这里id和name都可以是相同的，同一个字段里有相同的数字，相同的值 6. 也可以做一些限制，在插入相同的id的时候，就会冲突

update操作

更改db1.t1表的字符串为name 的数据和字符串为id 的数据
update db1.t1 set name='aaa' where id=1;

mysql> update db1.t1 set name='aaa' where id=1;
Query OK, 2 rows affected (0.01 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | aaa  |
|    1 | aaa  |
+------+------+
2 rows in set (0.00 sec)

mysql>

delete操作

删除db1.t1表的数据和字符串为id 的数据
delete from db1.t1 where id=1;

mysql> delete from db1.t1 where id=1;
Query OK, 2 rows affected (0.01 sec)

mysql> select * from db1.t1;
Empty set (0.00 sec)

mysql>

truncate清空一个表

清空表数据 truncate table db1.t1;
- 即使表的数据清空了，但表的字段依旧存在的

mysql> insert into db1.t1 values (1, 234);
Query OK, 1 row affected (0.00 sec)

mysql> select * from db1.t1;
+------+------+
| id   | name |
+------+------+
|    1 | 234  |
+------+------+
1 row in set (0.00 sec)

mysql> truncate table db1.t1;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from db1.t1;
Empty set (0.00 sec)

mysql> desc db1.t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(4)   | YES  |     | NULL    |       |
| name  | char(40) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql>

truncate 只是清空的内容，而drop 会清空表的数据并清除表的框架
drop 会把表的框架也丢掉 drop table db1.t1;

mysql> drop table db1.t1;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from db1.t1;    //因为表的架构已经不存在了
ERROR 1146 (42S02): Table 'db1.t1' doesn't exist
mysql>

丢掉表 drop database db1;

总结

在使用mysql的时候，少用 * 这样的操作，因为若是一个表里面的内容很多，select count(*)这样操作就会很耗时，浪费资源
数据库中常用引擎是myisam和innodb，默认mysql库里面都是使用的myisam引擎
- 特点：myisam引擎，能自动去统计有多少行
  - 在select count(*)查看表的时候会很快
  - use mysql;
  - show create table userG;
- 特点：innodb引擎，不会自动统计行数，每次去查询，每次去统计行数，就会很耗时
  - use db1
  - show create table t1;
- 所以select count(*)这种操作尽量减少，会耗费太多资源