MySQL "many to many"关系错误

Question

我有 3 个处于“多对多”关系的表。

表A:

CREATE TABLE IF NOT EXISTS tabA(
tabAcolA mediumint unsigned not null auto_increment,
tabAcolB text not null,
tabAcolC text null,
PRIMARY KEY keyA (tabAcolA)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表B:

CREATE TABLE IF NOT EXISTS tabB(
tabBcolA mediumint unsigned not null auto_increment,
tabBcolB text not null,
PRIMARY KEY keyB (tabBcolA)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表C(AB接头):

CREATE TABLE IF NOT EXISTS tabC(
    tabCcolA int unsigned not null,
    tabCcolB int unsigned not null,
    PRIMARY KEY keyC (tabCcolA,tabCcolB),
    CONSTRAINT FkeyA FOREIGN KEY (tabCcolA) 
    REFERENCES tabA(tabAcolA) ON DELETE RESTRICT ON UPDATE RESTRICT,
    CONSTRAINT FkeyB FOREIGN KEY (tabCcolB) 
    REFERENCES tabB(tabBcolA) ON DELETE RESTRICT ON UPDATE RESTRICT
    ) ENGINE=InnoDB

它返回一个错误:

1215 Cannot add foreign key constraint

哪里出了问题？

Answer 1

问题是列的数据类型不匹配。

外键列的数据类型必须与引用的列完全匹配。例如

 fkcol  INT UNSIGNED     ref -> pkcol  INT UNSIGNED

或

 fkcol  MEDIUMINT        ref -> pkcol  MEDIUMINT

如果我们尝试使用不同数据类型的列创建外键约束，InnoDB 将返回错误。 (OP观察到的行为。)