如何将主键作为外部引用到MySQL中的各种表？

alter table yourSecondTableName add constraint `yourConstraintName` foreign key(`yourSecondTableNamePrimaryKey`) references yourFirstTableName(yourFirstTablePrimaryKeyColumnName);

为了理解上述语法，让我们首先创建一个表 -

mysql> create table demo65
−> (
−> id int not null primary key,
−> name varchar(20)
−> );
Query OK, 0 rows affected (0.57 sec)

以下是创建第二个表的查询 -

mysql> create table demo66
−> (
−> user_id int not null primary key,
−> address varchar(200)
−> );
Query OK, 0 rows affected (1.80 sec)

以下是将主键引用为外键的查询 -

mysql> alter table demo66
−> add constraint `id_fk`
−> foreign key(`user_id`)
−> references demo65(id);
Query OK, 0 rows affected (3.76 sec)
Records: 0 Duplicates: 0 Warnings: 0

让我们使用 SHOW CREATE TABLE 命令检查表的整体描述。以下是查询 -

mysql> show create table demo66;

这将产生以下输出 -

+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table +--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| demo66 | CREATE TABLE `demo66` (
`user_id` int NOT NULL,
`address` varchar(200) DEFAULT NULL,
PRIMARY KEY (`user_id`),
CONSTRAINT `id_fk` FOREIGN KEY (`user_id`) REFERENCES `demo65` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

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