Question

我有一个递归的subject表，因为主题可能是另一个主题的先决条件：

主题没有先决条件
主题可以有1个先决条件
主题可以有2个先决条件

由于有两个先决条件的主题，我将prerequisite_1和prerequisite_2列FK指向subject_code列：

CREATE TABLE subject(
    subject_code CHAR(7),
    subject_desc VARCHAR(255) NOT NULL,
    no_of_units TINYINT UNSIGNED NOT NULL CHECK(no_of_units >= 0 AND no_of_units < 13),
    prerequisite_1 CHAR(7),
    prerequisite_2 CHAR(7),
    PRIMARY KEY(subject_code),
    FOREIGN KEY(prerequisite_1, prerequisite_2) REFERENCES subject(subject_code)
)ENGINE=INNODB;

我在SQL Fiddle上尝试了代码，并且没有创建表。

如何创建一个递归表，其中1个PK列和2个FK列指向表格自己的PK？

Answer 1

不要将先决条件放在subject表中，而是使用多对多关系表：

CREATE TABLE prerequisite (
    subject_code CHAR(7),
    prerequisite CHAR(7),
    PRIMARY KEY (subject_code, prerequisite),
    FOREIGN KEY (subject_code) REFERENCES subject(subject_code),
    FOREIGN KEY (prerequisite) REFERENCES subject(subject_code)
)

这允许任意数量的先决条件。

Answer 2

您需要将它们分成两个单独的外键，您应该（但不必）将它们命名;

CREATE TABLE subject(
    subject_code CHAR(7),
    subject_desc VARCHAR(255) NOT NULL,
    no_of_units TINYINT UNSIGNED NOT NULL 
       CHECK(no_of_units >= 0 AND no_of_units < 13),
    prerequisite_1 CHAR(7),
    prerequisite_2 CHAR(7),
    PRIMARY KEY(subject_code),
    FOREIGN KEY fk_pr1(prerequisite_1) REFERENCES subject(subject_code),
    FOREIGN KEY fk_pr2(prerequisite_2) REFERENCES subject(subject_code)
) ENGINE=INNODB;

An SQLfiddle

Answer 3

所以，这是我的评论：

我相信您的语法错误，请尝试将每个外键的foreign key内容更改为：

CREATE TABLE subject(
    .........

    CONSTRAINT `subject_ibfk_1` FOREIGN KEY (`prerequisite_1`) REFERENCES `subject` (`subject_code`),

    CONSTRAINT `subject_ibfk_2` FOREIGN KEY (`prerequisite_2`) REFERENCES `subject` (`subject_code`)

)ENGINE=INNODB;

SQL DDL：带有两个外键的递归表（MySQL）

3 个答案: