一文了解MySQL基础
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1、终端操作mysql数据库
1.1如何登陆数据库服务器：
C:Usersusername>mysql -uroot -proot
1.2如何查询数据库服务器中的数据库
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| test1 |
+--------------------+
5 rows in set (0.03 sec)
1.3如何选中某一个数据库进行操作
mysql> use test1
Database changed
1.4 SQL语句中的查询
mysql> select * from person;
+-----------+------+
| name | sex |
+-----------+------+
| 张三 | 男 |
| 李四 | 女 |
| 王麻子 | 男 |
+-----------+------+
3 rows in set (0.00 sec)
mysql> select * from person where name="张三";
+--------+------+
| name | sex |
+--------+------+
| 张三 | 男 |
+--------+------+
1 row in set (0.00 sec)
1.5如何退出数据库服务器
mysql> exit;
Bye
2、如何创建数据库数据表
2.1如何在数据库服务器中创建我们的数据库
mysql> create database test2;
Query OK, 1 row affected (0.05 sec)
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| test1 |
| test2 |
+--------------------+
6 rows in set (0.00 sec)
mysql> use test2;
Database changed
2.2如何查看某个数据库中的所有数据表
mysql> show tables;
Empty set (0.00 sec)
2.3如何创建一个数据表
mysql> create table pet(
-> name VARCHAR(20),
-> owner VARCHAR(20),
-> species VARCHAR(20),
-> sex CHAR(1),
-> birth DATE,
-> death DATE);
Query OK, 0 rows affected (0.09 sec)
创建了一个宠物数据表
查看数据表是否创建成功
mysql> show tables;
+-----------------+
| Tables_in_test2 |
+-----------------+
| pet |
+-----------------+
1 row in set (0.00 sec)
2.4查看创建好的数据表的结构
mysql> describe pet;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL | |
| owner | varchar(20) | YES | | NULL | |
| species | varchar(20) | YES | | NULL | |
| sex | char(1) | YES | | NULL | |
| birth | date | YES | | NULL | |
| death | date | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
3、insert增加数据记录
3.1查看数据表中的记录
mysql> select * from pet;
Empty set (0.00 sec)
3.2如何往数据表中添加数据记录
mysql> insert into pet
-> values('Tom','LiMing','hamster','f','2020-11-11',null);
Query OK, 1 row affected (0.01 sec)
再一次查询
mysql> select * from pet;
+------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+------+--------+---------+------+------------+-------+
| Tom | LiMing | hamster | f | 2020-11-11 | NULL |
+------+--------+---------+------+------------+-------+
1 row in set (0.00 sec)
mysql> insert into pet
-> values('旺财','周星星','狗','公','2020-11-11',null);
Query OK, 1 row affected (0.01 sec)
查看
mysql> select * from pet;
+--------+-----------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+--------+-----------+---------+------+------------+-------+
| Tom | LiMing | hamster | f | 2020-11-11 | NULL |
| 旺财 | 周星星 | 狗 | 公 | 2020-11-11 | NULL |
+--------+-----------+---------+------+------------+-------+
2 rows in set (0.00 sec)
3.3MySQL常用数据类型
MySQL支持多种类型，大致可以分为三类：数值、日期/时间和字符串(字符)类型。
•数值类型
如果数值超出数值类型的范围就会报错：
mysql> create table testType(
-> number tinyint);
Query OK, 0 rows affected (0.10 sec)
mysql> describe testType;
+--------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| number | tinyint | YES | | NULL | |
+--------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> insert into testType
-> values(128);
ERROR 1264 (22003): Out of range value for column 'number' at row 1
•日期和时间类型
•字符串类型
数据类型如何选择
日期：按照日期格式
数值和字符串：按照大小
4、数据记录增删查改操作
4.1插入：
mysql> insert into pet values('Buffy','Job','cat','m','2020-11-10',null);
Query OK, 1 row affected (0.01 sec)
mysql> insert into pet values('Slim','邓紫棋','dog','f','2019-11-11',null);
Query OK, 1 row affected (0.01 sec)
mysql> insert into pet
-> values('Fang','成龙','cat','m','2018-11-11','2020-10-10');
Query OK, 1 row affected (0.01 sec)
mysql> select * from pet;
+--------+-----------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+--------+-----------+---------+------+------------+------------+
| Tom | LiMing | hamster | f | 2020-11-11 | NULL |
| 旺财 | 周星星 | 狗 | 公 | 2020-11-11 | NULL |
| Buffy | Job | cat | m | 2020-11-10 | NULL |
| Slim | 邓紫棋 | dog | f | 2019-11-11 | NULL |
| Fang | 成龙 | cat | m | 2018-11-11 | 2020-10-10 |
+--------+-----------+---------+------+------------+------------+
5 rows in set (0.00 sec)
4.2如何删除数据
mysql> delete from pet where name='Tom';
Query OK, 1 row affected (0.01 sec)
再次查看
mysql> select * from pet;
+--------+-----------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+--------+-----------+---------+------+------------+------------+
| 旺财 | 周星星 | 狗 | 公 | 2020-11-11 | NULL |
| Buffy | Job | cat | m | 2020-11-10 | NULL |
| Slim | 邓紫棋 | dog | f | 2019-11-11 | NULL |
| Fang | 成龙 | cat | m | 2018-11-11 | 2020-10-10 |
+--------+-----------+---------+------+------------+------------+
4 rows in set (0.00 sec)
4.3如何修改数据
mysql> update pet set name='旺财财' where owner='周星星';
Query OK, 1 row affected (0.01 sec)
再次查看：
mysql> select * from pet;
+-----------+-----------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+-----------+-----------+---------+------+------------+------------+
| 旺财财 | 周星星 | 狗 | 公 | 2020-11-11 | NULL |
| Buffy | Job | cat | m | 2020-11-10 | NULL |
| Slim | 邓紫棋 | dog | f | 2019-11-11 | NULL |
| Fang | 成龙 | cat | m | 2018-11-11 | 2020-10-10 |
+-----------+-----------+---------+------+------------+------------+
4 rows in set (0.00 sec)
4.4总结：数据记录常见操作
增加：insert
删除：delete
修改：update
查询：select
5、MySQL建表约束
5.1主键约束 primary key
它能够唯一确定一张表中的一条记录，也就是我们通过给某个字段添加约束，就可以使得该字段不重复且不为空。
mysql> create table user(
-> id int primary key,
-> name varchar(20));
Query OK, 0 rows affected (0.08 sec)
mysql> describe user;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
再次插入id=1的记录
mysql> insert into user values(1,'李四');
ERROR 1062 (23000): Duplicate entry '1' for key 'user.PRIMARY'
id为空的时候
mysql> insert into user values(null,'李四');
ERROR 1048 (23000): Column 'id' cannot be null
5.2联合主键
mysql> create table user2(
-> id int,
-> name varchar(20),
-> password varchar(20),
-> primary key(id,name)
-> );
Query OK, 0 rows affected (0.06 sec)
插入记录
mysql> insert into user2 values(1,'zhangsan','123');
Query OK, 1 row affected (0.01 sec)
mysql> insert into user2 values(1,'zhangsan','234');
ERROR 1062 (23000): Duplicate entry '1-zhangsan' for key 'user2.PRIMARY'
mysql> insert into user2 values(2,'zhangsan','234');
Query OK, 1 row affected (0.01 sec)
联合主键只要主键字段之间加起来不重复就可以，但任何一个主键字段都不可以为空。
5.3自增约束auto_increment
mysql> create table user3(
-> id int primary key auto_increment,
-> name varchar(20)
-> );
Query OK, 0 rows affected (0.05 sec)
插入数据
mysql> insert into user3 (name) values('zhangsan');
Query OK, 1 row affected (0.01 sec)
查看
mysql> select * from user3;
+----+----------+
| id | name |
+----+----------+
| 1 | zhangsan |
+----+----------+
1 row in set (0.00 sec)
自增约束和主键约束搭配在一起使用时，它会自动给我们管去控制，例如：自动增加id。
5.4如果创建表的时候忘记创建主键约束怎么办？
mysql> create table user4(
-> id int,
-> name varchar(20));
Query OK, 0 rows affected (0.06 sec)
mysql> desc user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
修改表结构，增加主键约束 alter ... add
mysql> alter table user4 add primary key(id);
Query OK, 0 rows affected (0.12 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
如何删除呢？alter ...drop
mysql> alter table user4 drop primary key;
Query OK, 0 rows affected (0.10 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
使用modify，修改字段，添加约束
mysql> alter table user4 modify id int primary key;
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> describe user4;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
5.5唯一约束
约束修饰的字段的值不可以重复。
mysql> create table user5(
-> id int,
-> name varchar(20));
Query OK, 0 rows affected (0.06 sec)
mysql> desc user5;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> alter table user5 add unique(name);
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc user5;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> insert into user5 values(1,'zhangsan');
Query OK, 1 row affected (0.03 sec)
mysql> insert into user5 values(1,'zhangsan');
ERROR 1062 (23000): Duplicate entry 'zhangsan' for key 'user5.name'
name=lisi不能重复
mysql> insert into user5 values(1,'lisi');
Query OK, 1 row affected (0.01 sec)
mysql> create table user6(
-> id int,
-> name varchar(20),
-> unique(name));
Query OK, 0 rows affected (0.07 sec)
可以对id和name都添加唯一约束，这时候只要id+name不重复就可以添加记录。
mysql> create table user7(
-> id int,
-> name varchar(20),
-> unique(id,name));
Query OK, 0 rows affected (0.06 sec)
mysql> insert into user7 values(1,'zhangsan');
Query OK, 1 row affected (0.01 sec)
mysql> insert into user7 values(2,'zhangsan');
Query OK, 1 row affected (0.04 sec)
mysql> select * from user7;
+------+----------+
| id | name |
+------+----------+
| 1 | zhangsan |
| 2 | zhangsan |
+------+----------+
2 rows in set (0.00 sec)
如何删除唯一约束
mysql> alter table user6 drop index name;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc user6;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
通过modify添加唯一约束
mysql> alter table user6 modify name varchar(20) unique;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc user6;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | YES | UNI | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
总结：
• 建表的时候添加约束• 可以用alter... add添加• 可以用alter... modify添加约束• 删除用alter...drop
5.6非空约束
修饰的字段不能为空null
mysql> create table user8(
-> id int,
-> name varchar(20) not null);
Query OK, 0 rows affected (0.05 sec)
mysql> desc user8;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | NO | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
测试：如果name为空
mysql> insert into user8 (id)values(1);
ERROR 1364 (HY000): Field 'name' doesn't have a default value
正确添加不为空时
mysql> insert into user8 values(1,'lisi');
Query OK, 1 row affected (0.01 sec)
mysql> select *from user8;
+------+------+
| id | name |
+------+------+
| 1 | lisi |
+------+------+
1 row in set (0.00 sec)
如果只传name是可以的
mysql> insert into user8 (name)values('zhangsan');
Query OK, 1 row affected (0.01 sec)
mysql> select *from user8;
+------+----------+
| id | name |
+------+----------+
| 1 | lisi |
| NULL | zhangsan |
+------+----------+
2 rows in set (0.00 sec)
同样可以用alter ... add，alter ... modify来添加唯一约束。
5.7默认约束default
就是当我们插入字段值的时候，如果没有传值，就会使用默认值。
mysql> create table user9(
-> id int,
-> name varchar(20),
-> age int default 10
-> );
Query OK, 0 rows affected (0.06 sec)
查看表结构
mysql> desc user9;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| age | int | YES | | 10 | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
插入值时只插入id和name那么age使用默认值10，如果age也插入值那么就使用插入的值不使用默认值。
mysql> insert into user9(id,name) values (1,'lisi');
Query OK, 1 row affected (0.01 sec)
mysql> select *from user9;
+------+------+------+
| id | name | age |
+------+------+------+
| 1 | lisi | 10 |
+------+------+------+
1 row in set (0.00 sec)
5.8 外键约束
涉及到两个表：父表，子表。
班级表：
mysql> create table classes(
-> id int primary key,
-> name varchar(20)
-> );
Query OK, 0 rows affected (0.05 sec)
mysql> desc classes;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
插入值
mysql> insert into classes values(1,'yiban');
Query OK, 1 row affected (0.01 sec)
mysql> insert into classes values(2,'erban');
Query OK, 1 row affected (0.03 sec)
mysql> insert into classes values(3,'sanban');
Query OK, 1 row affected (0.01 sec)
mysql> select *from classes;
+----+--------+
| id | name |
+----+--------+
| 1 | yiban |
| 2 | erban |
| 3 | sanban |
+----+--------+
3 rows in set (0.00 sec)
学生表：
mysql> create table students(
-> id int primary key,
-> name varchar(20),
-> class_id int,
-> foreign key(class_id) references classes(id));
Query OK, 0 rows affected (0.06 sec)
mysql> desc students;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int | NO | PRI | NULL | |
| name | varchar(20) | YES | | NULL | |
| class_id | int | YES | MUL | NULL | |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
插入值
mysql> insert into students values(1001,'zhangsan',1);
Query OK, 1 row affected (0.01 sec)
mysql> insert into students values(1002,'zhangsan',2);
Query OK, 1 row affected (0.01 sec)
mysql> insert into students values(1003,'zhangsan',3);
Query OK, 1 row affected (0.04 sec)
mysql> select *from students;
+------+----------+----------+
| id | name | class_id |
+------+----------+----------+
| 1001 | zhangsan | 1 |
| 1002 | zhangsan | 2 |
| 1003 | zhangsan | 3 |
+------+----------+----------+
3 rows in set (0.00 sec)
父表classes中没有的数据值，是不可以在子表students中使用的。
父表中的记录被子表引用，是不可以被删除的。
mysql> delete from classes where id=3;
ERROR 1451 (23000): Cannot delete or update a parent row:
a foreign key constraint fails (`test2`.`students`,
CONSTRAINT `students_ibfk_1` FOREIGN KEY (`class_id`) REFERENCES `classes` (`id`))
数据库三大范式
1.第一范式 1NF
数据库表中的所有字段都是不可分割的原子值。
mysql> create table student(
-> id int primary key,
-> name varchar(20),
-> address varchar(30)
-> );
Query OK, 0 rows affected (0.12 sec)
mysql> insert into student values(1,'zhangsan','甘肃会宁');
Query OK, 1 row affected (0.02 sec)
mysql> insert into student values(2,'lisi','甘肃会宁');
Query OK, 1 row affected (0.01 sec)
mysql> insert into student values(3,'laowang','甘肃会宁');
Query OK, 1 row affected (0.01 sec)
mysql> select * from student;
+----+----------+--------------+
| id | name | address |
+----+----------+--------------+
| 1 | zhangsan | 甘肃会宁 |
| 2 | lisi | 甘肃会宁 |
| 3 | laowang | 甘肃会宁 |
+----+----------+--------------+
3 rows in set (0.00 sec)
address字段值还可以继续拆分的就不满足第一范式。
create table student2(
id int primary key,
name varchar(20),
country varchar(30),
privence varchar(30),
city varchar(30),
details varchar(30)
);
insert into student2 values(1,'张三','中国','甘肃','兰州','安宁区安宁街道30号');
insert into student2 values(2,'李四','中国','甘肃','兰州','安宁区安宁街道34号');
insert into student2 values(3,'王五','中国','甘肃','兰州','安宁区安宁街道80号');
mysql> select * from student2;
+----+--------+---------+----------+--------+----------------------------+
| id | name | country | privence | city | details |
+----+--------+---------+----------+--------+----------------------------+
| 1 | 张三 | 中国 | 甘肃 | 兰州 | 安宁区安宁街道30号 |
| 2 | 李四 | 中国 | 甘肃 | 兰州 | 安宁区安宁街道34号 |
| 3 | 王五 | 中国 | 甘肃 | 兰州 | 安宁区安宁街道80号 |
+----+--------+---------+----------+--------+----------------------------+
3 rows in set (0.00 sec)
范式：设计的越详细，对于某些实际操作可能更好，但不一定都有好处。一般要结合实际情况设计。
1.第二范式 2NF
第二范式必须是满足第一范式的前提下，要求，除主键外的每一列都必须完全依赖于主键。如果出现不完全依赖，只可能发生在联合主键的情况下。
create table myolder(
product_id int,
customer_id int,
product_name varchar(30),
customer_name varchar(30),
primary key(product_id,customer_id)
);
问题：除主键外的其他列，只依赖于主键的部分字段。不满足第二范式 解决办法是拆表。
create table myolder(
older_id int primary key,
product_id int,
customer_id int
);
create table product(
id int primary key,
name varchar(20)
);
create table customer(
id int primary key,
name varchar(20)
);
分成三个表之后就满足第二范式的设计！！！
1.第三范式 3NF
必须先满足第二范式， 除开主键列的其他列之间不能有传递依赖关系。例如：
create table myolder(
older_id int primary key,
product_id int,
customer_id int,
customer_phone varchar(15)
);
在这个表中，我们可以看到customer_phone和customer_id之间存在依赖关系，这样就不满足第三范式的要求；解决办法是：我们把customer_phone放到customer表中。如下：
create table customer(
id int primary key,
name varchar(20),
phone varchar(15)
);
sql的四种连接查询
内连接
inner join 或者join
外连接
左连接 left join 或者left outer join
右连接 right join 或者right outer join
完全外连接
full join 或者full outer join
创建两个表：
person表
id，name，cardID
1 张三 1
2 李四 3
3 王五 6
card表
id,name
1 饭卡
2 建行卡
3 农行卡
4 工商卡
5 邮政卡
并没有创建外键
--inner join查询（内连接）
SELECT * from person INNER JOIN card ON person.cardID=card.id;
id name cardid id name
1 张三 1 1 饭卡
2 李四 3 3 农行卡
--内连查询，其实就是两张表中的数据，通过某个字段相等查询出相关记录数据。
--left join左外连接
SELECT * FROM person LEFT JOIN card ON person.cardID=card.id;
+------+--------+--------+------+-----------+
| id | name | cardID | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
| 3 | 王五 | 6 | NULL | NULL |
+------+--------+--------+------+-----------+
3 rows in set (0.00 sec)
---左外连接，会把左边表里所有的数据取出来，而右边表里的数据，如果有相等的，就显示出来，如果没有就补null。
----right join右外连接
SELECT * FROM person RIGHT JOIN card ON person.cardID=card.id;
+------+--------+--------+------+-----------+
| id | name | cardID | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| NULL | NULL | NULL | 2 | 建行卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
| NULL | NULL | NULL | 4 | 工商卡 |
| NULL | NULL | NULL | 5 | 邮政卡 |
+------+--------+--------+------+-----------+
5 rows in set (0.00 sec)
---右外连接，会把右边表里所有的数据取出来，而左边表里的数据，如果有相等的，就显示出来，如果没有就补null。
--------------------------------------------------------------------
----full join全外连接
---MySQL不支持全外连接
---只能通过union来实现
mysql> SELECT * FROM person LEFT JOIN card ON person.cardID=card.id
-> UNION
-> SELECT * FROM person RIGHT JOIN card ON person.cardID=card.id;
+------+--------+--------+------+-----------+
| id | name | cardID | id | name |
+------+--------+--------+------+-----------+
| 1 | 张三 | 1 | 1 | 饭卡 |
| 2 | 李四 | 3 | 3 | 农行卡 |
| 3 | 王五 | 6 | NULL | NULL |
| NULL | NULL | NULL | 2 | 建行卡 |
| NULL | NULL | NULL | 4 | 工商卡 |
| NULL | NULL | NULL | 5 | 邮政卡 |
+------+--------+--------+------+-----------+
6 rows in set (0.00 sec)
事务
MySQL事务
mysql 中，事务其实是一个最小的不可分割的工作单元。事务能够保证一个业务的完整性。
比如我们的银行转账:
a -> -100
update user set money=money-100 where name='a' ;
b -> +100
update user set money=money+100 where name='b ';
实际的程序中，如果只有一条语句执行成功了，而另外一条没有执行成功，就会出现数据前后不一致。
多条sql语句，可能会有同时成功的要求，要么就同时失败。
MySQL中如何控制事务
1.MySQL是默认开启事务的（自动开启）
SELECT @@autocommit;
-------------
@@autocommit
1
-------------
默认事务开启的作用是什么?
当我们去执行一个sql语句的时候，效果会立即体现出来，且不能回滚。
先创建一个表：
create database bank;
create table user(
id int primary key,
name varchar(20),
money int
);
insert into user values( 1, 'a' ,1000);
事务回滚：撤销sql语句执行效果。rollback;
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
mysql> select *from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 1000 |
+----+------+-------+
1 row in set (0.00 sec)
设置mysql自动提交为false。
mysql> set autocommit = 0;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@autocommit;
+--------------+
| @@autocommit |
+--------------+
| 0 |
+--------------+
1 row in set (0.00 sec)
上面的操作，关闭了mysql的自动提交(commit)。
我们再插入一条记录：
mysql> insert into user values(2,'b',1000);
Query OK, 1 row affected (0.00 sec)
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 1000 |
| 2 | b | 1000 |
+----+------+-------+
2 rows in set (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 1000 |
+----+------+-------+
1 row in set (0.00 sec)
插入第二条数据其实没有插入到实际表中；
要想插入成功，则进行如下操作：
再次插入
mysql> insert into user values(2,'b',1000);
Query OK, 1 row affected (0.00 sec)
手动提交数据
mysql> commit;
Query OK, 0 rows affected (0.01 sec)
撤销不可以（持久性）
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 1000 |
| 2 | b | 1000 |
+----+------+-------+
2 rows in set (0.00 sec)
总结：
自动提交：@@autocommit
手动提交：commit
事务回滚：rollback
例如转账操作：
mysql> update user set money=money-100 where name='a' ;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update user set money=money+100 where name='b ';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 900 |
| 2 | b | 1000 |
+----+------+-------+
2 rows in set (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 1000 |
| 2 | b | 1000 |
+----+------+-------+
2 rows in set (0.00 sec)
事务给我们提供了一个返回的机会。
手动开启事务
两种方式
begin;
start transaction;
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> update user set money=money-100 where name='a';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update user set money=money+100 where name='b ';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 800 |
| 2 | b | 1000 |
+----+------+-------+
2 rows in set (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from user;
+----+------+-------+
| id | name | money |
+----+------+-------+
| 1 | a | 900 |
| 2 | b | 1000 |
+----+------+-------+
2 rows in set (0.00 sec)
事务开启之后，一旦commit提交，就不能回滚了（也就是当前的这个事务在提交的时候就结束了）。
事务的一些基本概念
事务的四大特征ACID
A 原子性：事务是最小的单位，不可以在分割。
C 一致性：事务要求，同一事务中的sql语句，必须保证同时成功或者同时失败。
I 隔离性：事务1和事务2之间是具有隔离性的。
D 持久性：事务一旦结束（commit，rollback），就不可以返回。
事务开启
1。修改默认提交set autocommit=0;
1.begin;2.start transaction;
事务手动提交
commit;
事务手动回滚
rollback;
事务的隔离性
1、read uncommitted;
读未提交的
2、read committed;
读已经提交的
3、repeatable read;
可以重复读
4、serializable;
串行化
mysql 默认隔离级别REPEATABLE-READ
解释：
如果有事务a,和事务b,
a事务对数据进行操作，在操作的过程中，事务没有被提交，但是b 可以看见a操作的结果。
eg：bank数据库的user表：
insert into user values( 3,'小明',1000);
insert into user values(4,'淘宝店' ,1000) ;
如何查看数据库的隔离级别：
系统级别：select @@global.transaction_isolation;
会话级别：select @transaction_isolation;
mysql> select @@global.transaction_isolation;
+--------------------------------+
| @@global.transaction_isolation |
+--------------------------------+
| REPEATABLE-READ |
+--------------------------------+
1 row in set (0.00 sec)
如何修改隔离级别：
set global transaction isolation level read uncommitted;
mysql> set global transaction isolation level read uncommitted;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@global.transaction_isolation;
+--------------------------------+
| @@global.transaction_isolation |
+--------------------------------+
| READ-UNCOMMITTED |
+--------------------------------+
1 row in set (0.00 sec)
转账:小明在淘宝店买鞋子:800块钱,
小明-》成都ATM
淘宝店-》广州ATM
start transaction;
update user set money=money-800 where name='小明';
update user set money=money+800 where name='淘宝店';
mysql> select * from user;
+----+-----------+-------+
| id | name | money |
+----+-----------+-------+
| 1 | a | 900 |
| 2 | b | 1000 |
| 3 | 小明 | 200 |
| 4 | 淘宝店 | 1800 |
+----+-----------+-------+
4 rows in set (0.00 sec)
如果两个不同的地方，都在进行操作，事务a 开启之后，他的数据可以被其他事务读取到。这样就会出现(脏读)。
脏读：一个事务读到了另外一个事务没有提交的数据，就叫做脏读。
实际开发是不允许脏读出现的。
不可重复读 虽然我只能读到另外一个事务提交的数据，但还是会出现问题，就是读取同一个表的数据，发现前后不一致。不可重复读现象。
幻读事务a和事务b 同时操作一张表，事务a提交的数据，也不能被事务b读到，就可以造成幻读。
串行化 问题是，性能特差!!!
READ—UNCOMMITTED > READ-COMMITTED > REPEATABLE-READ > SERIALIZABLE;
隔离级别越高，性能越差。
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