[英]Can the same column have primary key & foreign key constraint to another column
同一列可以對另一列具有主鍵和外鍵約束嗎?
Table1: ID - Primary column, foreign key constraint for Table2 ID
Table2: ID - Primary column, Name
如果我嘗試刪除 table1 數據,這會是一個問題嗎?
Delete from table1 where ID=1000;
謝謝。
將主鍵和外鍵分配給表中的同一列:
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);
應該沒有問題。 考慮以下示例:
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);
這些表現在包含:
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)
現在我們可以像這樣成功刪除行:
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,我們將無法刪除以下內容:
DELETE FROM table2 WHERE id = 1;
ERROR 1451 (23000): A foreign key constraint fails
如果我們使用CASCADE
選項在table1
上定義外鍵,我們將能夠刪除父級,並且所有子級都會被自動刪除:
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);
如果我們重復之前失敗的DELETE
,則table1
的子行以及table2
的父行將被刪除:
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)
是的,它可以。
不,不會。
PS但是如果不刪除相應的table1行,您將無法刪除table2數據。
PPS 我已經在 Postgres 中實現了這樣的結構,但它對於 MySQL 必須是類似的。
Jason 提供的答案可能在過去一段時間有效,但是當我嘗試在 2021 年針對 MySQL 5.7 服務器使用此答案時,它抱怨。 我用來讓這個工作的語法是;
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);
對於這種類型的一對一關系,我還強烈建議您將外鍵創建為;
CONSTRAINT `fk_id1` FOREIGN KEY (id1) REFERENCES a1(id1) ON DELETE CASCADE
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