同一列可以对另一列有主键和外键约束吗

Question

同一列可以对另一列有主键和外键约束吗？

Table1: ID - Primary column, foreign key constraint for Table2 ID
Table2: ID - Primary column, Name 

如果我尝试删除 table1 数据，这会是一个问题吗？

Delete from table1 where ID=1000;

谢谢。

Answer 1

将主键和外键分配给表中的同一列：

create table a1 (
    id1 int not null primary key 
);
insert into a1 values(1),(2),(3),(4);

create table a2 (
    id1 int not null primary key foreign key references a1(id1)
);
insert into a2 values(1),(2),(3);

Answer 2

应该没有问题。考虑以下示例：

CREATE TABLE table2 (
   id int PRIMARY KEY,
   name varchar(20)
) ENGINE=INNODB;

CREATE TABLE table1 (
   id int PRIMARY KEY, 
   t2_id int, 
   FOREIGN KEY (t2_id) REFERENCES table2 (id)
) ENGINE=INNODB;

INSERT INTO table2 VALUES (1, 'First Row');
INSERT INTO table2 VALUES (2, 'Second Row');

INSERT INTO table1 VALUES (1, 1);
INSERT INTO table1 VALUES (2, 1);
INSERT INTO table1 VALUES (3, 1);
INSERT INTO table1 VALUES (4, 2);

这些表格现在包含：

SELECT * FROM table1;
+----+-------+
| id | t2_id |
+----+-------+
|  1 |     1 |
|  2 |     1 |
|  3 |     1 |
|  4 |     2 |
+----+-------+
4 rows in set (0.00 sec)

SELECT * FROM table2;
+----+------------+
| id | name       |
+----+------------+
|  1 | First Row  |
|  2 | Second Row |
+----+------------+
2 rows in set (0.00 sec)

现在我们可以像这样成功删除行了：

DELETE FROM table1 WHERE id = 1;
Query OK, 1 row affected (0.00 sec)

DELETE FROM table1 WHERE t2_id = 2;
Query OK, 1 row affected (0.00 sec)

但是我们无法删除以下内容：

DELETE FROM table2 WHERE id = 1;
ERROR 1451 (23000): A foreign key constraint fails

如果我们在table1 上使用CASCADE 选项定义外键，我们将能够删除父级，并且所有子级都将被自动删除：

CREATE TABLE table2 (
   id int PRIMARY KEY,
   name varchar(20)
) ENGINE=INNODB;

CREATE TABLE table1 (
   id int PRIMARY KEY, 
   t2_id int, 
   FOREIGN KEY (t2_id) REFERENCES table2 (id) ON DELETE CASCADE
) ENGINE=INNODB;

INSERT INTO table2 VALUES (1, 'First Row');
INSERT INTO table2 VALUES (2, 'Second Row');

INSERT INTO table1 VALUES (1, 1);
INSERT INTO table1 VALUES (2, 1);
INSERT INTO table1 VALUES (3, 1);
INSERT INTO table1 VALUES (4, 2);

如果我们要重复之前失败的DELETE，table1 中的子行以及table2 中的父行将被删除：

DELETE FROM table2 WHERE id = 1;
Query OK, 1 row affected (0.00 sec)

SELECT * FROM table1;
+----+-------+
| id | t2_id |
+----+-------+
|  4 |     2 |
+----+-------+
1 row in set (0.00 sec)

SELECT * FROM table2;
+----+------------+
| id | name       |
+----+------------+
|  2 | Second Row |
+----+------------+
1 row in set (0.00 sec)

Answer 3

是的，可以。

不，不会。

附注但是如果不删除相应的table1行，您将无法删除table2数据。

P.P.S.我已经在 Postgres 中实现了这样的结构，但是对于 MySQL 来说它必须是类似的。

Answer 4

Jason 提供的答案可能在过去一段时间内有效，但当我在 2021 年尝试将此答案用于 MySQL 5.7 服务器时，它会抱怨。我用来完成这项工作的语法是；

CREATE TABLE a1 (
    id1 INT NOT NULL PRIMARY KEY
);
INSERT INTO a1 VALUES (1),(2),(3),(4);

CREATE TABLE a2 (
    id1 INT NOT NULL,
    PRIMARY KEY (id1),
    CONSTRAINT `fk_id1` FOREIGN KEY (id1) REFERENCES a1(id1)
);
INSERT INTO a2 VALUES (1),(2),(3);

对于这种类型的一对一关系，我也强烈建议您将外键创建为；

CONSTRAINT `fk_id1` FOREIGN KEY (id1) REFERENCES a1(id1) ON DELETE CASCADE