您可以在一条语句中访问 MySQL 中的自动增量值吗?
我有一个包含用户表的 MySQL 数据库.表的主键是'userid',设置为自增字段.
I have a MySQL database which contains a table of users. The primary key of the table is 'userid', which is set to be an auto increment field.
我想要做的是,当我向表中插入新用户时,使用与自动增量在不同字段default_assignment"中的userid"字段中创建的值相同的值.
What I'd like to do is when I insert a new user into the table is to use the same value that the auto increment is creating in the 'userid' field in a different field, 'default_assignment'.
例如
我想要这样的声明:
INSERT INTO users ('username','default_assignment') VALUES ('barry', value_of_auto_increment_field())
所以我创建了用户 'Barry',生成的 'userid' 为 16(例如),但我也希望 'default_assignment' 具有相同的值 16.
so I create user 'Barry', the 'userid' is generated as being 16 (for example), but I also want the 'default_assignment' to have the same value of 16.
请问有什么办法可以实现吗?
Is there any way to achieve this please?
谢谢!
更新:
感谢您的回复.default_assignment 字段不是多余的.default_assigment 可以引用 users 表中的任何用户.创建用户时,我已经有一个允许选择另一个用户作为 default_assignment 的表单,但是在某些情况下需要将其设置为同一用户,因此我的问题是.
Thanks for the replies. The default_assignment field isn't redundant. The default_assigment can reference any user within the users table. When creating a user I already have a form that allows a selection of another user as the default_assignment, however there are cases where it needs to be set to the same user, hence my question.
更新:
好的,我已经尝试了更新触发器建议,但仍然无法让它起作用.这是我创建的触发器:
Ok, I've tried out the update triggers suggestion but still can't get this to work. Here's the trigger I've created:
CREATE TRIGGER default_assignment_self BEFORE INSERT ON `users`
FOR EACH ROW BEGIN
SET NEW.default_assignment = NEW.userid;
END;
当插入一个新用户时,default_assignment 总是设置为 0.
When inserting a new user however the default_assignment is always set to 0.
如果我手动设置用户 ID,那么 default_assignment 会设置为用户 ID.
If I manually set the userid then the default_assignment does get set to the userid.
因此,自动分配生成过程显然发生在触发器生效之后.
Therefore the auto assignment generation process clearly happens after the trigger takes effect.
推荐答案
不需要另外创建表,max()根据表的auto_increment值会有问题,这样做:
there's no need to create another table, and max() will have problems acording to the auto_increment value of the table, do this:
CREATE TRIGGER trigger_name BEFORE INSERT ON tbl FOR EACH ROW
BEGIN
DECLARE next_id;
SET next_id = (SELECT AUTO_INCREMENT FROM information_schema.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='tbl');
SET NEW.field = next_id;
END
我声明 next_id 变量,因为通常它会以其他方式使用(*),但您可以直接使用 new.field=(select ...)
I declare the next_id variable because usually it will be used in some other way(*), but you could do straight new.field=(select ...)
CREATE TRIGGER trigger_name BEFORE INSERT ON tbl FOR EACH ROW
BEGIN
SET NEW.field=(SELECT AUTO_INCREMENT FROM information_schema.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='tbl');
END
同样在(SELECT 字符串字段)的情况下,您可以使用 CAST 值;
Also in cases of (SELECT string field) you can use CAST value;
CREATE TRIGGER trigger_name BEFORE INSERT ON tbl FOR EACH ROW
BEGIN
SET NEW.field=CAST((SELECT aStringField FROM information_schema.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='tbl') AS UNSIGNED);
END
(*) 要自动命名图像:
(*) To auto-name an image:
SET NEW.field = CONCAT('image_', next_id, '.gif');
(*) 创建一个散列:
(*) To create a hash:
SET NEW.field = CONCAT( MD5( next_id ) , MD5( FLOOR( RAND( ) *10000000 ) ) );
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