问题
I have table of users in my MySQL database. Is it possible to set default value of column profile_url to value of column Id ?
回答1:
You could use trigger to implement the feature. For example:
CREATE TRIGGER defaultUrl
BEFORE INSERT ON MyTable
FOR EACH ROW
SET NEW.profile_url = IFNULL(NEW.profile_url, NEW.id);
The expression creates a trigger named defaultUrl that activates before each row inserted into MyTable. If defaultUrl is NULL, the id value is used.
来源：https://stackoverflow.com/questions/53250756/mysql-set-dynamic-default-value