MySQL - MySQL 8.0(三)进阶操作：生成列（generated column）

此学习文是基于MySQL 8.0写的
得益于大神朋友的悉心指导解决不少坑，才写出此文，向大神奉上膝盖

  生成列（generated column）的值是根据列定义中包含的表达式计算得出的。 生成列包含下面两种类型：

• virtual（虚拟）：当从表中读取记录时，将动态计算该列。
• stored（存储）：当向表中写入新记录时，将计算该列并将其存储为常规列。

  virtual生成列比stored生成列更有用，因为一个虚拟的列不占用任何存储空间。你可以使用触发器模拟stored生成列的行为。

举个栗子

1. 创建表时

  假设你的应用程序从t_employees_generated表中检索数据时，使用full_name表示concat(first_name, ' ', last_name)，而不是使用表达式来表示，从而实现虚拟列实时计算full_name。

# 创建测试表
mysql> create table if not exists employees.t_employees_generated (
    -> emp_no int(11) not null,
    -> birth_date date not null,
    -> first_name varchar(14) not null,
    -> last_name varchar(16) not null,
    -> gender enum('M', 'F') not null,
    -> hire_date date not null,
    -> full_name varchar(30) as (concat(first_name, ' ', last_name)),
    -> primary key (emp_no),
    -> key name (first_name, last_name)
    -> ) engine=innodb default charset=utf8mb4;
Query OK, 0 rows affected, 1 warning (0.04 sec)

  请注意，应该根据虚拟列修改插入语句。你可以这样使用full insert：

# 执行插入
mysql> insert into employees.t_employees_generated 
    -> (emp_no, birth_date, first_name, last_name, gender, hire_date) 
    -> values 
    -> (123456, '1987-10-02', 'ABC', 'XYZ', 'F', '2008-07-28');
Query OK, 1 row affected (0.11 sec)

# 验证数据
mysql> select * from employees.t_employees_generated where emp_no = '123456';
+--------+------------+------------+-----------+--------+------------+-----------+
| emp_no | birth_date | first_name | last_name | gender | hire_date  | full_name |
+--------+------------+------------+-----------+--------+------------+-----------+
| 123456 | 1987-10-02 | ABC        | XYZ       | F      | 2008-07-28 | ABC XYZ   |
+--------+------------+------------+-----------+--------+------------+-----------+
1 row in set (0.00 sec)

  如果要在INSERT语句中包含full_name，就只能将其指定为DEFAULT

# 执行插入
mysql> insert into employees.t_employees_generated 
    -> (emp_no, birth_date, first_name, last_name, gender, hire_date, full_name) 
    -> values 
    -> (123457, '1987-10-02', 'ABC', 'XYZ', 'F', '2008-07-28', DEFAULT);
Query OK, 1 row affected (0.01 sec)

# 验证数据
mysql> select * from employees.t_employees_generated where emp_no = '123457';
+--------+------------+------------+-----------+--------+------------+-----------+
| emp_no | birth_date | first_name | last_name | gender | hire_date  | full_name |
+--------+------------+------------+-----------+--------+------------+-----------+
| 123457 | 1987-10-02 | ABC        | XYZ       | F      | 2008-07-28 | ABC XYZ   |
+--------+------------+------------+-----------+--------+------------+-----------+
1 row in set (0.00 sec)

  其他值都会引发ERROR 3105 (HY000):错误，不允许在t_employees_generated表中为生成的列full_name指定值：

mysql> insert into employees.t_employees_generated 
    -> (emp_no, birth_date, first_name, last_name, gender, hire_date, full_name) 
    -> values 
    -> (123458, '1987-10-02', 'ABC', 'XYZ', 'F', '2008-07-28', 'TEST');
ERROR 3105 (HY000): The value specified for generated column 'full_name' in table 't_employees_generated' is not allowed.

2. 已有表时

  如果你已经创建了表并希望添加新的生成列，请执行ALTER TABLE语句

# 修改表结构 - 新增生成列
mysql> alter table employees.t_employees_generated add hire_date_year year as (year(hire_date)) virtual;
Query OK, 0 rows affected (0.12 sec)
Records: 0  Duplicates: 0  Warnings: 0

# 验证数据
mysql> select * from employees.t_employees_generated where emp_no = '123456';
+--------+------------+------------+-----------+--------+------------+-----------+----------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date  | full_name | hire_date_year |
+--------+------------+------------+-----------+--------+------------+-----------+----------------+
| 123456 | 1987-10-02 | ABC        | XYZ       | F      | 2008-07-28 | ABC XYZ   |           2008 |
+--------+------------+------------+-----------+--------+------------+-----------+----------------+
1 row in set (0.00 sec)

# 查看表结构
mysql> desc employees.t_employees_generated;
+----------------+---------------+------+-----+---------+-------------------+
| Field          | Type          | Null | Key | Default | Extra             |
+----------------+---------------+------+-----+---------+-------------------+
| emp_no         | int(11)       | NO   | PRI | NULL    |                   |
| birth_date     | date          | NO   |     | NULL    |                   |
| first_name     | varchar(14)   | NO   | MUL | NULL    |                   |
| last_name      | varchar(16)   | NO   |     | NULL    |                   |
| gender         | enum('M','F') | NO   |     | NULL    |                   |
| hire_date      | date          | NO   |     | NULL    |                   |
| full_name      | varchar(30)   | YES  |     | NULL    | VIRTUAL GENERATED |
| hire_date_year | year(4)       | YES  |     | NULL    | VIRTUAL GENERATED |
+----------------+---------------+------+-----+---------+-------------------+
8 rows in set (0.00 sec)

详情参阅 https://dev.mysql.com/doc/refman/8.0/en/create-table-generated-columns.html