create table TEST ( col_1 INT, col_2 INT, col_3 VARCHAR(128) not null, col_4 VARCHAR(128) not null, UNIQUE KEY (col_1, col_2, col_3), CONSTRAINT FOREIGN KEY (col_1) REFERENCES T2(ID) ON DELETE CASCADE, CONSTRAINT FOREIGN KEY (col_2) REFERENCES T3(ID) ON DELETE CASCADE ) comment='测试表' ENGINE=InnoDB DEFAULT CHARACTER SET=utf8;
本意是要求如下:
INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //OK INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //ERROR
但是实际上MYSQL却可以插入数据,没有提示违反了UNIQUE KEY (col_1, col_2, col_3),这个是MYSQL对NULL处理的方法引起的。虽然很多用户在给MYSQL提了N个关于这个的BUG,但是MYSQL死抱着SQL specs不放,就是不改或者提供可选的解决方案。
那究竟有没方法可以满足我的本意呢?有人提供了这样的一个方法:
create table TEST ( ID INT not null AUTO_INCREMENT, col_1 INT, col_2 INT, col_3 VARCHAR(128) not null, col_4 VARCHAR(128) not null, PRIMARY KEY (ID), CONSTRAINT FOREIGN KEY (col_1) REFERENCES T2(ID) ON DELETE CASCADE, CONSTRAINT FOREIGN KEY (col_2) REFERENCES T3(ID) ON DELETE CASCADE ) comment='测试表' ENGINE=InnoDB DEFAULT CHARACTER SET=utf8;
首先,去掉了组合的唯一索引,而是加入了一个自增长PK:ID
其次,为此表创建一个triigger:
CREATE TRIGGER `TEST_BEFORE_INS` BEFORE INSERT ON `TEST` FOR EACH ROW BEGIN SET NEW.id=(SELECT id FROM TEST WHERE col_1<=>NEW.col_1 AND col_2<=>NEW.col_2 AND col_3<=>NEW.col_3); END
再来测试下:
INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //OK INSERT INTO TEST (col_1, col_2, col_3, col_4) VALUES (1, NULL, 'TEST', 'VALID'); //ERROR:Duplicate entry '1' for key 'PRIMARY'
报主键重复错误,看来,基本满足了我的本意需求。