MySQL: 创建外键时出错，似乎正确时出现错误 150

Question

我必须创建一个数据库，这是 MySQL 代码：

    drop database if EXISTS BibliotecaLello;
create database BibliotecaLello;
use BibliotecaLello; 

create table Lettore
(
    CodiceFiscale varchar(17) PRIMARY KEY not null  
);

create TABLE Tessera 
(
    CodiceLettore varchar(17),
    CodiceTessera integer PRIMARY KEY not null AUTO_INCREMENT,
    Nome varchar(10) not null,
    Cognome varchar(10) not null,
    unique(Nome, Cognome)
);

create table Autori
(
    IDAutore integer PRIMARY KEY NOT null AUTO_INCREMENT,
    Nome varchar(20) not null,
    Cognome varchar(20) not null
);

create table Libro
(
    CodiceLibro integer AUTO_INCREMENT PRIMARY KEY not null,
    IDAutore integer not null,
    Titolo varchar(20) not null
);

create table Copia
(
    CodiceLibro integer not null,
    CodiceCopia integer not null,
    unique(CodiceLibro, CodiceCopia)
);

create table Noleggio
(
    CodiceLettore varchar(17) not null,
    DataInizio date not null,
    DataFine date not null,
    CodiceLibro integer not null,
    CodiceCopia integer not null,
    unique(CodiceLettore, DataInizio, DataFine, CodiceLibro, CodiceCopia)
);

alter table Noleggio add CONSTRAINT fkNolleggioLettore
FOREIGN key (CodiceLettore)
REFERENCES `Lettore`(`CodiceFiscale`) on DELETE CASCADE on UPDATE CASCADE;

alter table Tessera add CONSTRAINT fkTesseraLettori
FOREIGN KEY (`CodiceLettore`) 
REFERENCES `Lettore`(`CodiceFiscale`) on DELETE CASCADE on UPDATE cascade;

alter table Libro add CONSTRAINT fkLibroAutori
FOREIGN KEY (IDAutore)
REFERENCES `Autori`(`IDAutore`)  on DELETE CASCADE on UPDATE cascade;

alter table `Copia` add CONSTRAINT fkCopieLibro
FOREIGN KEY (CodiceLibro) 
REFERENCES Libro(CodiceLibro)  on DELETE CASCADE on UPDATE cascade;

alter table Noleggio add CONSTRAINT fkNoleggioCopiaCopia
FOREIGN KEY (CodiceCopia) 
REFERENCES `Copia`(`CodiceCopia`) on DELETE CASCADE on UPDATE cascade;

alter table Noleggio add CONSTRAINT fkNoleggioCopiaLibro
FOREIGN KEY (CodiceLibro) 
REFERENCES `Copia`(`CodiceLibro`) on DELETE CASCADE on UPDATE cascade;

但是当我执行

alter table Noleggio add CONSTRAINT fkNoleggioCopiaCopia
FOREIGN KEY (CodiceCopia) 
REFERENCES `Copia`(`CodiceCopia`) on DELETE CASCADE on UPDATE cascade;

我明白了

ERROR 1005 (HY000): Can't create table `BibliotecaLello`.`#sql-2f7_2c2` (errno: 150 "Foreign key constraint is incorrectly formed")

我试过调查

SHOW ENGINE INNODB STATUS

这是最新外键错误

的结果

2019-12-02 14:37:25 75ea2340 Error in foreign key constraint of table `BibliotecaLello`.`Noleggio`:

FOREIGN KEY (CodiceCopia) 
REFERENCES `Copia`(`CodiceCopia`) on DELETE CASCADE on UPDATE cascade:
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.

我的老师告诉我，这意味着我正在修改错误的 table，但是尝试使用两个 table 仍然无效。你能帮我解决这个问题吗？

Answer 1

当我运行这段代码时：

alter table Noleggio add CONSTRAINT fkNoleggioCopiaCopia
FOREIGN KEY (CodiceCopia) 
REFERENCES `Copia`(`CodiceCopia`) on DELETE CASCADE on UPDATE cascade;

它给我错误：

Failed to add the foreign key constraint. Missing index for constraint 'fkNoleggioCopiaCopia' in the referenced table 'Copia'

问题在于，在引用的 table 中，列 CodiceCopia 被定义为 UNIQUE 键中的第二列：

create table Copia
(
    CodiceLibro integer not null,
    CodiceCopia integer not null,
    unique(CodiceLibro, CodiceCopia)
);

这是一个 documented behavior:

Foreign key constraints are subject to the following conditions and restrictions:

MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order.

一个简单的解决方案是使其成为约束中的第一列，如下所示：

create table Copia
(
    CodiceLibro integer not null,
    CodiceCopia integer not null,
    unique(CodiceCopia, CodiceLibro)
);

旁注：最好在 table 上有一个合适的主键。

Demo on DB Fiddle

MySQL: 创建外键时出错，似乎正确时出现错误 150

MySQL: Error while creating foreign key, error 150 when it seems right

mysql

sql

database

foreign-keys

create-table