此學習文是基於MySQL 8.0寫的
得益於大神朋友的悉心指導解決不少坑,才寫出此文,向大神奉上膝蓋
生成列(generated column)的值是根據列定義中包含的表達式計算得出的。 生成列包含下面兩種類型:
virtual
(虛擬):當從表中讀取記錄時,將動態計算該列。stored
(存儲):當向表中寫入新記錄時,將計算該列並將其存儲爲常規列。
virtual
生成列比stored
生成列更有用,因爲一個虛擬的列不佔用任何存儲空間。你可以使用觸發器模擬stored
生成列的行爲。
舉個栗子
1. 創建表時
假設你的應用程序從t_employees_generated
表中檢索數據時,使用full_name
表示concat(first_name, ' ', last_name)
,而不是使用表達式來表示,從而實現虛擬列實時計算full_name
。
# 創建測試表
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)
請注意,應該根據虛擬列修改插入語句。你可以這樣使用full insert
:
# 執行插入
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)
# 驗證數據
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
語句中包含full_name
,就只能將其指定爲DEFAULT
# 執行插入
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)
# 驗證數據
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)
其他值都會引發ERROR 3105 (HY000):
錯誤,不允許在t_employees_generated
表中爲生成的列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. 已有表時
如果你已經創建了表並希望添加新的生成列,請執行ALTER TABLE
語句
# 修改表結構 - 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
# 驗證數據
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)
# 查看錶結構
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)
詳情參閱 https://dev.mysql.com/doc/refman/8.0/en/create-table-generated-columns.html