SQL语句实践

1. DML语句之管理表中的数据

1、往表中插入数据

在插入数据前，先新建一个简单的测试表test：

create table test(id int(4) NOT NULL AUTO_INCREMENT, name char(20) NOT NULL, PRIMARY KEY(id));
desc test;

往表中插入数据有几种不同的语法
第一种：按规则制定所有的列名，并且每列都插入值：

insert into test(id,name) values(1,'oldboy');

第二种：只在name列插入值，由于id列是自增的，所以可以这样操作：

insert into test(name) values('oldgirl');

也可以执行下面的语句来实现，注意，这两种情况只选择一条执行：

insert into test(id,name) values(null,'zhangsan');

第三种：如果不指定列，就要按规则为每列都插入恰当的值：

insert into test values(3,'inca');

第四种：批量插入数据的方法，可提升效率：

insert into test values(4,'zuma'),(5,'kaka');

下面是插入数据的实践：

mysql> use oldboy;
Database changed
mysql> drop table test;
Query OK, 0 rows affected (0.03 sec)
mysql> create table test(id int(4) NOT NULL AUTO_INCREMENT, name char(20) NOT NULL, PRIMARY KEY(id));
Query OK, 0 rows affected (0.06 sec)
mysql> desc test;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| id    | int(4)   | NO   | PRI | NULL    | auto_increment |
| name  | char(20) | NO   |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

通过以下命令插入第一行，请注意语法的细微不同：

mysql> insert into test(id,name) values(1,'oldboy');
Query OK, 1 row affected (0.02 sec)
mysql> insert into test(name) values('oldgirl');
Query OK, 1 row affected (0.02 sec)
mysql> insert into test values(3,'inca');
Query OK, 1 row affected (0.04 sec)
mysql> insert into test values(4,'zuma'),(5,'kaka');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0
mysql> select * from test;    ---查看插入后得结果
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
5 rows in set (0.00 sec)
mysql> delete from test;    ---全部删除
Query OK, 5 rows affected (0.00 sec)
mysql> select * from test;
Empty set (0.00 sec)
mysql> INSERT INTO `test` VALUES (1,'oldboy'),(2,'oldgirl'),(3,'inca'),(4,'zuma'),(5,'kaka');    ---批量重新插入
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0
mysql> select * from test;                                                             
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
5 rows in set (0.00 sec)

退出数据库，备份上述数据，留着备用：

mysql> quit
Bye
[root@oldboy ~]# mysqldump -uroot -poldboy123 -B oldboy >/opt/bak.sql
Warning: Using a password on the command line interface can be insecure.

备份后检查一下备份的SQL数据内容，同时过滤无用信息：

[root@oldboy ~]# egrep -v "#|\/\^$|--|^\/" /opt/bak.sql 
CREATE DATABASE /*!32312 IF NOT EXISTS*/ `oldboy` /*!40100 DEFAULT CHARACTER SET utf8 */;
USE `oldboy`;
DROP TABLE IF EXISTS `test`;
CREATE TABLE `test` (
  `id` int(4) NOT NULL AUTO_INCREMENT,
  `name` char(20) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;
LOCK TABLES `test` WRITE;
INSERT INTO `test` VALUES (1,'oldboy'),(2,'oldgirl'),(3,'inca'),(4,'zuma'),(5,'kaka');
UNLOCK TABLES;

可以看到上面备份的语句，就是我们执行的语句内容

2、查询表中的数据

查看表test中的所有数据，可采用如下两种方法
方法1：进入指定库后再查询（常用）：

mysql> use oldboy
Database changed
mysql> select * from test;
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
5 rows in set (0.00 sec)

方法2：直接在库外，使用“库.表”的方式查询：

mysql> select * from oldboy.test;    ---用点号分隔库和表，其中，这里的"*"表示所有字段
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
5 rows in set (0.00 sec)

可通过以下命令查询mysql库user表对应user和host列的用户：

mysql> select user,host from mysql.user;
+--------+---------------------------+
| user   | host                      |
+--------+---------------------------+
| root   | 127.0.0.1                 |
| blog   | 192.168.9.%               |
| test   | 192.168.9.%               |
| test1  | 192.168.9.%               |
| test   | 192.168.9.0/255.255.255.0 |
| root   | localhost                 |
| system | localhost                 |
| test   | localhost                 |
+--------+---------------------------+
8 rows in set (0.03 sec)

查看表test中的前2行数据：

mysql> select * from test limit 2;
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
+----+---------+
2 rows in set (0.00 sec)
mysql> select * from test limit 0,3;    ---从第0行开始查，查3行记录
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
+----+---------+
3 rows in set (0.00 sec)

查询id为1的行：

mysql> select * from test where id=1;    ---查询数字列无需加引号，但是看起来是数字，实际属于字符列就需要加引号了
+----+--------+
| id | name   |
+----+--------+
|  1 | oldboy |
+----+--------+
1 row in set (0.00 sec)
mysql> select * from test where name='oldgirl';    ---查询字符串必须加引号
+----+---------+
| id | name    |
+----+---------+
|  2 | oldgirl |
+----+---------+
1 row in set (0.00 sec)

若要同时查询多个条件，则取交集（and），例如查询id为2，并且name为oldgirl的命令如下：

mysql> select * from test where id=2 and name='oldgirl';
+----+---------+
| id | name    |
+----+---------+
|  2 | oldgirl |
+----+---------+
1 row in set (0.00 sec)

指定固定条件范围查询数据：

mysql> select id,name from test where id>2 and id<5;    ---多个条件，and取交集
+----+------+
| id | name |
+----+------+
|  3 | inca |
|  4 | zuma |
+----+------+
2 rows in set (0.00 sec)
mysql> select id,name from test where id>3 or id<2;    ---多个条件，or取并集
+----+--------+
| id | name   |
+----+--------+
|  1 | oldboy |
|  4 | zuma   |
|  5 | kaka   |
+----+--------+
3 rows in set (0.00 sec)

查询排序功能：

mysql> select id,name from test where id>3 order by id asc;    ---order by ... asc为顺序排序
+----+------+
| id | name |
+----+------+
|  4 | zuma |
|  5 | kaka |
+----+------+
2 rows in set (0.00 sec)
mysql> select id,name from test where id<3 order by id desc;    ---order by ... desc为倒序排序
+----+---------+
| id | name    |
+----+---------+
|  2 | oldgirl |
|  1 | oldboy  |
+----+---------+
2 rows in set (0.00 sec)

3、修改表中的数据

先查看要修改的表：

mysql> select * from test;
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
5 rows in set (0.00 sec)

将id为3的行的名字修改为gongli：

mysql> update test set name='gongli' where id=3;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0
mysql> select * from test where id=3;
+----+--------+
| id | name   |
+----+--------+
|  3 | gongli |
+----+--------+
1 row in set (0.00 sec)

4、修改数据导致的事故案例和解决方案

如果不带条件更改所有表的记录：

mysql> update test set name='xiaoting';    ---如果不加条件则要十分小心。专业做法是要多向开发人员确认，发送给开发人员的语句一定要括号注明，以防DBA误会。
Query OK, 5 rows affected (0.00 sec)
Rows matched: 5  Changed: 5  Warnings: 0
mysql> select * from test;    ---可以发现，所有得name列的内容都是相同的了
+----+----------+
| id | name     |
+----+----------+
|  1 | xiaoting |
|  2 | xiaoting |
|  3 | xiaoting |
|  4 | xiaoting |
|  5 | xiaoting |
+----+----------+
5 rows in set (0.00 sec)

面对上面的故障，开始用之前备份的数据进行恢复（备份的重要性）：

mysql> drop table test;
Query OK, 0 rows affected (0.03 sec)
mysql> source /opt/bak.sql
Query OK, 0 rows affected (0.03 sec)
mysql> select * from test;
+----+---------+
| id | name    |
+----+---------+
|  1 | oldboy  |
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
5 rows in set (0.00 sec)

5、删除表中得数据

删除表test中编号为1的记录：

mysql> use oldboy
Database changed
mysql> delete from test where id=1;
Query OK, 1 row affected (0.00 sec)
mysql> delete from test where name='oldboy';
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+----+---------+
| id | name    |
+----+---------+
|  2 | oldgirl |
|  3 | inca    |
|  4 | zuma    |
|  5 | kaka    |
+----+---------+
4 rows in set (0.00 sec)

提示：

• 不加任何条件（where）就是全部删除，这也是非常危险的操作。对于不加条件的delete、update，一定要慎用回车键，按照流程操作（运维人员和开发人员一起操作）
• 开发人员在程序里可能不用delete语句，而是用update语句来更新显示的状态（实现逻辑删除）

有些企业开发人员在开发程序时，会通过状态来判断页面内容显示。比如，在test表中有如下数据，环境接上下文：

mysql> alter table test add state tinyint(2) not null default 1;
Query OK, 0 rows affected (0.15 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> desc test;
+-------+------------+------+-----+---------+----------------+
| Field | Type       | Null | Key | Default | Extra          |
+-------+------------+------+-----+---------+----------------+
| id    | int(4)     | NO   | PRI | NULL    | auto_increment |
| name  | char(20)   | NO   |     | NULL    |                |
| state | tinyint(2) | NO   |     | 1       |                |
+-------+------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

然后，程序显示内容时，使用如下语句：

mysql> select * from test where state=1;
+----+---------+-------+
| id | name    | state |
+----+---------+-------+
|  2 | oldgirl |     1 |
|  3 | inca    |     1 |
|  4 | zuma    |     1 |
|  5 | kaka    |     1 |
+----+---------+-------+
4 rows in set (0.00 sec)

如果要删除数据，就可以用update替代delete实现逻辑删除，现在删除oldgirl所在的行：

mysql> update test set state=0 where name='oldgirl';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

查看删除结果，可以发现oldgirl 所在的行确实消失了，但实际上数据表里依然有，只不过状态为0，从而不显示了：

mysql> select * from test where state=1;
+----+------+-------+
| id | name | state |
+----+------+-------+
|  3 | inca |     1 |
|  4 | zuma |     1 |
|  5 | kaka |     1 |
+----+------+-------+
3 rows in set (0.00 sec)

提示：
网页正常显示的数据SQL语句为“select * from test where state=1;”删除上述oldgirl的记录语句为“update test set state=0 where name='oldgirl';”因此实际上数据并未真的删除，而是显示状态变为0了，如果想要真正删除，还可以写个在夜里定时任务清理state为0的状态行。

6、清空表中的数据

mysql> select * from test;    ---清空前查看下
+----+---------+-------+
| id | name    | state |
+----+---------+-------+
|  2 | oldgirl |     0 |
|  3 | inca    |     1 |
|  4 | zuma    |     1 |
|  5 | kaka    |     1 |
+----+---------+-------+
4 rows in set (0.00 sec)
mysql> truncate table test;    ---执行清空命令
Query OK, 0 rows affected (0.03 sec)
mysql> select * from test;    ---查询发现数据为空了
Empty set (0.00 sec)

truncate和delete是有区别的：

• TRANCATE与不带WHERE语句的DELETE功能相同：两者均删除表中的全部行，但TRUNCATE比DELETE速度更快
• TRANCATE通过释放存储表数据所用的数据页来删除数据，并且只在事务日志中记录页的释放，因此使用的系统和事务日志资源更少
• DELETE语句每次删除一行，并且会在事务日志中为所删除的每一行记录一项