如何在MySQL中为列设置默认值？
要设置默认值，请使用DEFAULT关键字。
让我们首先创建一个表-
create table DemoTable758 ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, FirstName varchar(100) );
以下是为列设置默认值的查询-
alter table DemoTable758 add column Colors ENUM('RED','GREEN','BLUE','ORANGE','YELLOW') DEFAULT 'YELLOW';
Records: 0 Duplicates: 0 Warnings: 0让我们再次检查表的描述-
desc DemoTable758;
这将产生以下输出-
+-----------+----------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+----------------------------------------------+------+-----+---------+----------------+
| Id | int(11) | NO | PRI | NULL | auto_increment |
| FirstName | varchar(100) | YES | | NULL | |
| Colors | enum('RED','GREEN','BLUE','ORANGE','YELLOW') | YES | | YELLOW | |
+-----------+----------------------------------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)使用insert命令在表中插入一些记录。在这里，我们没有使用名字“John”插入第二列的值。默认值“YELLOW”将放置在此处-
insert into DemoTable758(FirstName) values('John');
insert into DemoTable758(FirstName,Colors) values('Carol','RED');使用select语句显示表中的所有记录-
select *from DemoTable758;
这将产生以下输出-
+----+-----------+--------+ | Id | FirstName | Colors | +----+-----------+--------+ | 1 | John | YELLOW | | 2 | Carol | RED | +----+-----------+--------+ 2 rows in set (0.00 sec)