MySQL中的OR约束或非null约束

Question

在MySQL中创建非NULL约束以使fieldA和fieldB不能都为NULL的最佳方法是什么。我不在乎任何一个本身是否为NULL，只要另一个字段具有非NULL值即可。而且，如果它们都具有非NULL值，那就更好了。

Answer 1

MySQL
5.5引入了SIGNAL，因此我们不再需要Bill
Karwin的答案中的额外列。Bill指出您还需要一个更新触发器，因此我也将其包括在内。

CREATE TABLE foo (
  FieldA INT,
  FieldB INT
);

DELIMITER //
CREATE TRIGGER InsertFieldABNotNull BEFORE INSERT ON foo
FOR EACH ROW BEGIN
  IF (NEW.FieldA IS NULL AND NEW.FieldB IS NULL) THEN
    SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = '\'FieldA\' and \'FieldB\' cannot both be null';
  END IF;
END//
CREATE TRIGGER UpdateFieldABNotNull BEFORE UPDATE ON foo
FOR EACH ROW BEGIN
  IF (NEW.FieldA IS NULL AND NEW.FieldB IS NULL) THEN
    SIGNAL SQLSTATE '45000'
    SET MESSAGE_TEXT = '\'FieldA\' and \'FieldB\' cannot both be null';
  END IF;
END//
DELIMITER ;

INSERT INTO foo (FieldA, FieldB) VALUES (NULL, 10); -- OK
INSERT INTO foo (FieldA, FieldB) VALUES (10, NULL); -- OK
INSERT INTO foo (FieldA, FieldB) VALUES (NULL, NULL); -- gives error
UPDATE foo SET FieldA = NULL; -- gives error