我试过了
alter table t_granja ALTER COLUMN purchase_date TIMESTAMP DEFAULT now();
但是我收到了一个错误
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'TIMESTAMP DEFAULT now()' at line 1
使用CURRENT_TIMESTAMP
ALTER TABLE t_granja
ALTER COLUMN purchase_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP;