1. 概述

数据库，顾名思义用来存放数据的地方，可以把数据库比作图书馆，图书馆里面有不同区域相当于不同的数据库（database），而每个区域又存放着不同的书籍，相当于每个database含有很多表（table），图中表现了这种关系。

而表中有包含数据，数据是由记录（行）和字段（列）构成的，每条记录的字段的数据就是存储的数据。表即普通的表格，下表中有两条记录（两行），三个字段（三列）为id，name，age。

id	name	age
0	superman	32
1	wonderwoman	102

对于数据库，表，记录，字段都有增、删、改、查的操作，而sql（Structured Query Language）即结构化查询语言就是用来完成这些操作的，对于不同的对象有着不同的sql语句。大致包含对数据库的操作，对表的操作，对字段的操作，对记录的操作。在执行单条sql语句时可以不以;结束，但是执行多条语句时需要以;结束。而且sql语句大小写不敏感，即不区分大小写。

2. 数据库操作sql

2.1 查

show databases：查询当前所有数据库

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
4 rows in set (0.00 sec)

use 数据库名：连接到指定的数据库
1
2
mysql> use mysql;
Database changed

show tables：查看该数据库中的所有表

mysql> show tables;
+------------------------------------------------------+
| Tables_in_mysql                                      |
+------------------------------------------------------+
| columns_priv                                         |
| component                                            |
| db                                                   |
| default_roles                                        |
...
| engine_cost                                          |
| func                                                 |
| time_zone_name                                       |
+------------------------------------------------------+
35 rows in set (0.00 sec)

2.2 增

create database 数据库名：创建数据库

mysql> create database mydatabase;
Query OK, 1 row affected (1.93 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydatabase         |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)

2.3 删

drop database 数据库名：删除数据库

mysql> drop database mydatabase;
Query OK, 0 rows affected (2.01 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
4 rows in set (0.00 sec)

2.4 改

数据库名称没有直接修改命令，只能重新创建新库然后导入旧库

3. 表操作sql

3.1 增

create table：创建表
语法：

CREATE TABLE 表名
(
字段名1 数据类型,
字段名2 数据类型,
字段名3 数据类型,
...
)

数据库的基本数据类型可以分为三种：数字型，字符串型，日期型，而数字型又有整型和小数。由于每一种数据库的数据类型不太一样，以mysql为例

mysql> create table dc_character(
       id int,
       name varchar(255),
       age int)
Query OK, 0 rows affected (2.60 sec)

3.2 查

desc 表名：查看表详情

mysql> desc dc_character;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int          | YES  |     | NULL    |       |
| name  | varchar(255) | YES  |     | NULL    |       |
| age   | int          | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

3.3 改

alter table 旧表名 rename to 新表名：修改表名

mysql> alter table dc_character rename to dc;
Query OK, 0 rows affected (2.17 sec)

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

3.4 删

drop table 表名：删除表

mysql> drop table dc;
Query OK, 0 rows affected (2.14 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydatabase         |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
5 rows in set (0.00 sec)

3.5 字段操作sql

字段即表的列属性，而表字段本身包含5个属性：
1. Field：字段的名称
2. Type：字段数据类型
3. Null：字段的数据是否可以为Null，取值可以是null或者not null，后者就表示字段取值不可以是空
4. KEY：字段是否是主键或外来键，取值有primary key，unique Key, key 和 foreign Key
5. Default：字段的默认值
6. Extra：字段的一些额外信息

3.5.1 增

创建字段时可以加入属性，基本语法为

1	字段名字段类型 [Null取值] [KEY取值] [Default 默认值] [Extra信息]

比如

mysql> create table dc(
  -> id int primary key auto_increment,
  -> name varchar(255) not null,
  -> age int default 30);
Query OK, 0 rows affected (1.35 sec)

mysql> desc dc;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int          | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
| age   | int          | YES  |     | 30      |                |
+-------+--------------+------+-----+---------+----------------+

创建了表，其中id字段设置为主键，并设置为自增；name字段设置数据不能为Null；age字段设置默认值为30。

3.5.2 查

查询表的字段信息与前面的表操作查询一样

3.5.3 改

alter table 表名 add 新字段名数据类型：增加字段

mysql> alter table dc add power bigint;
Query OK, 0 rows affected (2.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc dc;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int          | YES  |     | NULL    |       |
| name  | varchar(255) | YES  |     | NULL    |       |
| age   | int          | YES  |     | NULL    |       |
| power | bigint       | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

alter table 表名称 change 旧字段名新字段名新字段类型：修改字段

mysql> alter table dc change power power_level int;
Query OK, 2 rows affected (2.36 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> desc dc;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| id          | int          | YES  |     | NULL    |       |
| name        | varchar(255) | YES  |     | NULL    |       |
| age         | int          | YES  |     | NULL    |       |
| power_level | int          | YES  |     | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

alter table 表名称 modify 字段名称新字段类型：只修改字段类型

mysql> alter table dc modify power_level bigint;
Query OK, 2 rows affected (2.56 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> desc dc;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| id          | int          | YES  |     | NULL    |       |
| name        | varchar(255) | YES  |     | NULL    |       |
| age         | int          | YES  |     | NULL    |       |
| power_level | bigint       | YES  |     | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

3.5.4 删

alter table 表名 drop 字段名：删除字段

ysql> alter table dc drop power_level;
Query OK, 0 rows affected (1.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc dc;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int          | YES  |     | NULL    |       |
| name  | varchar(255) | YES  |     | NULL    |       |
| age   | int          | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

3.6 数据操作sql

假设表dc的结构为

mysql> desc dc;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int          | NO   | PRI | NULL    | auto_increment |
| name       | varchar(255) | NO   |     | NULL    |                |
| age        | int          | YES  |     | 30      |                |
| powerlevel | int          | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

3.6.1 查

查询表数据是最重要，也是最常用的sql语句。

3.6.1.1 基本语法

select 字段1 [as 别名1], 字段2 [as 别名2], ... from 表名 [where 条件] [order by 字段名 [desc|asc]] [limit 条数]：从表中选择满足条件的数据的指定字段，再按照某个字段排序后并限制前面多少条输出

mysql> select * from dc; -- 最简单的查询语句，查询所有记录，* 表示所有字段
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   32 |          8 |
|  3 | batman      |   35 |          4 |
|  4 | flash       |   30 |          7 |
|  5 | lantern     |   30 |          6 |
|  6 | supergirl   |   33 |          8 |
|  7 | cyborg      |   32 |          5 |
+----+-------------+------+------------+
7 rows in set (0.00 sec)

mysql> select * from dc where age>=35; -- 加入条件查询年龄age大于等于25的记录
+----+----------+------+------------+
| id | name     | age  | powerlevel |
+----+----------+------+------------+
|  1 | superman |   35 |         10 |
|  3 | batman   |   35 |          4 |
+----+----------+------+------------+
2 rows in set (0.00 sec)

mysql> select name, age, powerlevel from dc where age>= 35 order by powerlevel; -- 在上一个查询基础上加入对结果按powerlevel排序，默认按升序（asc），加入关键词desc，可以使结果按降序
+----+----------+------+------------+
| id | name     | age  | powerlevel |
+----+----------+------+------------+
|  3 | batman   |   35 |          4 |
|  1 | superman |   35 |         10 |
+----+----------+------+------------+
2 rows in set (0.00 sec)

mysql> select * from dc limit 3; -- 查询记录只显示前面3条
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   32 |          8 |
|  3 | batman      |   35 |          4 |
+----+-------------+------+------------+
3 rows in set (0.00 sec)

3.6.1.2 聚合语法

select [字段1, 字段2, ...] 聚合函数3(字段3) [ [as] 别名3], 聚合函数4(字段4) [ [as] 别名4], ... from 表名 [group by 字段1, 字段2, ...]：对某几个字段聚合，即相同的归一类，然后对其他字段求聚合函数，比如最大最小值等

mysql> select max(powerlevel) from dc where age<35; -- 不加group by 对所有记录取聚合函数
+-----------------+
| max(powerlevel) |
+-----------------+
|               8 |
+-----------------+
1 row in set (0.00 sec)

mysql> select age, max(powerlevel) as mp from dc group by age order by mp desc; -- 查看年龄相同的数据中分别最大的powerlevel，并按照最大值降序排序，取别名 as 可以省略，直接 max(powerlevel) mp
+------+------+
| age  | mp   |
+------+------+
|   35 |   10 |
|   32 |    8 |
|   33 |    8 |
|   30 |    7 |
+------+------+
4 rows in set (0.00 sec)

3.6.1.3 多表联合查询

有时候需要联合多个表的数据来统计结果，这时需要用到join语句，与前面两种语法相比，只需要改变from后面的语法即可。

select ... from 表1 [ [as] 别名1 ] [left|right|outer|inner] join 表2 [ [as] 别名2 ] on ([表1|别名1].字段1=[表2|别名2].字段1 [and 表1.字段1=表2.字段1]...)：基于相同字段的值联合多个表查询

假设新增一个表dc_name，其数据为

mysql> select * from dc_name;
+------+------------+------+
| id   | name       | iq   |
+------+------------+------+
|    1 | lex luthor |  100 |
|    2 | batman     |  100 |
+------+------------+------+
2 rows in set (0.00 sec)

inner join

表示两个表取交集，即两个表都存在的数据取出

select * from dc t1 inner join dc_name t2 on (t1.name=t2.name); -- 默认会输出两个表的所有字段，若只写join，则默认是inner join
+----+--------+------+------------+------+--------+------+
| id | name   | age  | powerlevel | id   | name   | iq   |
+----+--------+------+------------+------+--------+------+
|  3 | batman |   35 |          4 |    2 | batman |  100 |
+----+--------+------+------------+------+--------+------+
1 row in set (0.00 sec)

mysql> select t1.*, t2.iq from dc t1 inner join dc_name t2 on (t1.name=t2.name); -- 只要t1的所有字段，t2的iq字段
+----+--------+------+------------+------+
| id | name   | age  | powerlevel | iq   |
+----+--------+------+------------+------+
|  3 | batman |   35 |          4 |  100 |
+----+--------+------+------------+------+
1 row in set (0.00 sec)

outer join

表示两个表取并集，即两个表存在的数据都会取出，全称 full outer join，不过有些mysql版本不支持，具体可以使用下面介绍到的union语法替代

left join

左表的数据不会丢失，将右表的数据添加到左表中；即左表有而右边没有的数据填null，而左表没有右表有的数据丢弃，左右表都有的数据直接添加

mysql> select t1.*, t2.iq from dc t1 left join dc_name t2 on (t1.name=t2.name); -- 一般left join 或者 right join 会加入where条件过滤，比如where iq is not null，则效果相当于inner join的效果
+----+-------------+------+------------+------+
| id | name        | age  | powerlevel | iq   |
+----+-------------+------+------------+------+
|  1 | superman    |   35 |         10 | NULL |
|  2 | wonderwoman |   32 |          8 | NULL |
|  3 | batman      |   35 |          4 |  100 |
|  4 | flash       |   30 |          7 | NULL |
|  5 | lantern     |   30 |          6 | NULL |
|  6 | supergirl   |   33 |          8 | NULL |
|  7 | cyborg      |   32 |          5 | NULL |
+----+-------------+------+------------+------+
7 rows in set (0.00 sec)

right join

与left join正好相反，右表的数据不会丢失

mysql> select t1.*, t2.iq from dc t1 right join dc_name t2 on (t1.name=t2.name);
+------+--------+------+------------+------+
| id   | name   | age  | powerlevel | iq   |
+------+--------+------+------------+------+
| NULL | NULL   | NULL |       NULL |  100 |
|    3 | batman |   35 |          4 |  100 |
+------+--------+------+------------+------+
2 rows in set (0.00 sec)

注意：2个以上表的join语法

1
2
3

select * from a
join b on (a.a1=b.b1)
join c on (a.a1=c.c1)

3.6.1.4 子查询

以上语法中from语法后的表名都可以替换为select查询语句得到的中间结果，相当于从中间查询结果中再次查询，具体语法是

select ... from (select ... from ...) ...：嵌套查询，从中间查询结果中再次查询，一般会将中间查询结果与原表join后再查询

mysql> select dc.* from dc inner join (select age, max(powerlevel) as mp from dc group by age) t1 on (dc.age=t1.age and dc.powerlevel=t1.mp); -- 查询年龄age相同的数据中，powerlevel最大的记录并包含名字name等所有原始信息的数据
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   32 |          8 |
|  4 | flash       |   30 |          7 |
|  6 | supergirl   |   33 |          8 |
+----+-------------+------+------------+
4 rows in set (0.00 sec)

3.6.1.5 查询结果合并

union语法，可以合并多个select语句的查询结果，具体语法：

select ... from ... [distinct] union select ... from ...

mysql> select * from dc t1 left join dc_name t2 on (t1.name=t2.name)
    -> union
    -> select * from dc t1 right join dc_name t2 on (t1.name=t2.name);  -- 使用union语句合并left join与right join的结果，相当于full outer join
+------+-------------+------+------------+------+------------+------+
| id   | name        | age  | powerlevel | id   | name       | iq   |
+------+-------------+------+------------+------+------------+------+
|    1 | superman    |   35 |         10 | NULL | NULL       | NULL |
|    2 | wonderwoman |   32 |          8 | NULL | NULL       | NULL |
|    3 | batman      |   35 |          4 |    2 | batman     |  100 |
|    4 | flash       |   30 |          7 | NULL | NULL       | NULL |
|    5 | lantern     |   30 |          6 | NULL | NULL       | NULL |
|    6 | supergirl   |   33 |          8 | NULL | NULL       | NULL |
|    7 | cyborg      |   32 |          5 | NULL | NULL       | NULL |
| NULL | NULL        | NULL |       NULL |    1 | lex luthor |  100 |
+------+-------------+------+------------+------+------------+------+
8 rows in set (0.00 sec)

3.6.2 增

insert into 表名 (字段1, 字段2, ...) values (字段1数据, 字段2数据, ...)：向表中添加一行数据

mysql> insert into dc values (0, 'superman', 35, 10);
Query OK, 1 row affected (0.08 sec)

mysql> select * from dc;
+----+----------+------+------------+
| id | name     | age  | powerlevel |
+----+----------+------+------------+
|  1 | superman |   35 |         10 |
+----+----------+------+------------+
1 row in set (0.00 sec)

注意到:

如果不填字段名称，默认所有字段按顺序匹配
由于id是自增的，所以id默认从1开始，添加数据的时候也可以指定字段，这样id会自动加1。

mysql> insert into dc (name, age) values ('wonderwoman', 34);
Query OK, 1 row affected (0.12 sec)

mysql> select * from mydatabase.dc;
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   34 |       NULL |
+----+-------------+------+------------+
2 rows in set (0.00 sec)

注意到id自增1，powerlevel会取默认值NULL。

insert (into|overwrite) 表名 select查询语句：从另一表中中导入数据

into是追加方式
overwirte是重写方式

mysql> create table dc_name (id int, name varchar(255)); -- 创建新表dc_name
Query OK, 0 rows affected (2.56 sec)

mysql> show tables;
+----------------------+
| Tables_in_mydatabase |
+----------------------+
| dc                   |
| dc_name              |
+----------------------+
2 rows in set (0.00 sec)

mysql> insert into dc_name select id, name from dc; -- 从dc表中导入数据给新表dc_name
Query OK, 2 rows affected (0.92 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from dc_name;
+------+-------------+
| id   | name        |
+------+-------------+
|    1 | superman    |
|    2 | wonderwoman |
+------+-------------+
2 rows in set (0.00 sec)

3.6.3 删

delete from 表名 [where 条件]：根据条件从表中删除数据

mysql> delete from dc_name where id=1; -- 删除id=1的记录
Query OK, 1 row affected (0.10 sec)

mysql> select * from dc_name;
+------+-------------+
| id   | name        |
+------+-------------+
|    2 | wonderwoman |
+------+-------------+
1 row in set (0.00 sec)

mysql> delete from dc_name; -- 不加条件，默认删除所有记录
Query OK, 1 row affected (0.08 sec)

mysql> select * from dc_name;
Empty set (0.00 sec)

3.6.4 改

update 表名 set 字段1=值1 [, 字段2=值2, ...] where 条件：修改某个数据

mysql> select * from dc;
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   34 |       NULL |
+----+-------------+------+------------+
2 rows in set (0.00 sec)

mysql> update dc set age=30, powerlevel=8 where id=2; -- 修改id=2的age字段与powerlevel字段
Query OK, 1 row affected (0.08 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from dc;
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   30 |          8 |
+----+-------------+------+------------+
2 rows in set (0.00 sec)

mysql> update dc set age=35; -- 不加where条件，默认修改所有行的age字段值
Query OK, 1 row affected (1.94 sec)
Rows matched: 2  Changed: 1  Warnings: 0

mysql> select * from dc;
+----+-------------+------+------------+
| id | name        | age  | powerlevel |
+----+-------------+------+------------+
|  1 | superman    |   35 |         10 |
|  2 | wonderwoman |   35 |          8 |
+----+-------------+------+------------+
2 rows in set (0.00 sec)

尘雨尘风

数据库之sql实践