列值自动递增，取决于另一个列值Entity Framework Core

Question

I want to have one more column which will autoincrement depending on another column value:

ID    |   UserID   |  IDForUser
 1    |     4      |     1
 2    |     10     |     1
 3    |     4      |     2
 4    |     4      |     3
 5    |     10     |     2
 6    |     9      |     1
 7    |     9      |     2

Is that possible?

Answer 1

The built-in autoincrementing algorithm does not work like this.

To compute the next IDForUser value, search for the largest existing value with the same UserID (or zero), and add one. If you do not want to do this in your code, you must use a trigger :

CREATE TRIGGER xxx
AFTER INSERT ON MyTable
FOR EACH ROW
WHEN NEW.IDForUser IS NULL
BEGIN
    UPDATE MyTable
    SET IDForUser = IFNULL((SELECT MAX(IDForUser)
                            FROM MyTable
                            WHERE UserID = NEW.UserID), 0) + 1
    WHERE ID = NEW.ID;
END;