MySQL 數據類型之枚舉類型

• Enum枚舉類型是字符串類型，其值是從事先指定的一系列值中選出，適用在某列的取值范圍已經固定
• 主要好處為MySQL在存儲此類數據時，直接轉化成數字存儲而不是字符串，可以節省空間，並且在表的.frm文件中存儲“數字-字符串”之間的對應關系

mysql> CREATE TABLE shirts (
    -> name VARCHAR(40),
    -> size ENUM('x-small', 'small', 'medium', 'large', 'x-large')
    -> );
Query OK, 0 rows affected (0.14 sec)

mysql> insert into shirts values('a','x-small');
Query OK, 1 row affected (0.05 sec)

mysql> select * from shirts;
+------+---------+
| name | size    |
+------+---------+
| a    | x-small |
+------+---------+
1 row in set (0.00 sec)

mysql> insert into shirts values('a','x-small2');
ERROR 1265 (01000): Data truncated for column 'size' at row 1
mysql> select * from shirts;
+------+---------+
| name | size    |
+------+---------+
| a    | x-small |
+------+---------+
1 row in set (0.01 sec)

mysql> truncate table shirts;
Query OK, 0 rows affected (0.26 sec)

mysql> INSERT INTO shirts (name, size) VALUES ('dress shirt','large'), ('t-shirt','medium'),
    -> ('polo shirt','small');
Query OK, 3 rows affected (0.04 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from shirts;                                                              
+-------------+--------+
| name        | size   |
+-------------+--------+
| dress shirt | large  |
| t-shirt     | medium |
| polo shirt  | small  |
+-------------+--------+
3 rows in set (0.00 sec)

mysql> create table temp2(size varchar(10));
ERROR 1050 (42S01): Table 'temp2' already exists
mysql> drop table temp2;
Query OK, 0 rows affected (0.14 sec)

mysql> create table temp2(size varchar(10),size varchar(10));
ERROR 1060 (42S21): Duplicate column name 'size'
mysql> create table temp2(name varchar(10),size varchar(10));
Query OK, 0 rows affected (0.16 sec)

mysql> alter table temp2 modify name varchar(20);
Query OK, 0 rows affected (0.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into temp2 values('dress shirt','large'), ('t-shirt','medium'), ('polo shirt',,'small');
Query OK, 3 rows affected (0.08 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from temp2;
+-------------+--------+
| name        | size   |
+-------------+--------+
| dress shirt | large  |
| t-shirt     | medium |
| polo shirt  | small  |
+-------------+--------+
3 rows in set (0.00 sec)

mysql> desc temp2;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(20) | YES  |     | NULL    |       |
| size  | varchar(10) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.01 sec)

mysql> select * from temp2 order by size;
+-------------+--------+
| name        | size   |
+-------------+--------+
| dress shirt | large  |
| t-shirt     | medium |
| polo shirt  | small  |
+-------------+--------+
3 rows in set (0.00 sec)

mysql> select * from shirts order by size;
+-------------+--------+
| name        | size   |
+-------------+--------+
| polo shirt  | small  |
| t-shirt     | medium |
| dress shirt | large  |
+-------------+--------+
3 rows in set (0.00 sec)