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以下是語法 –

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

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為了理解上述語法，讓我們首先創建一個表 –

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

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以下是創建第二個表的查詢 –

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

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以下是將主鍵引用為外鍵的查詢 –

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

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讓我們使用 SHOW CREATE TABLE 命令檢查表的整體描述。以下是查詢 –

mysql> show create table demo66;

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這將產生以下輸出 –

+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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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