How to refer primary key as Foreign to various table in MySQL?

Following is the syntax −

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

To understand the above syntax, let us first create a table −

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

Following is the query to create second table −

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

Following is the query to refer primary key as a foreign key −

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

Let us check the overall description of the table using SHOW CREATE TABLE command. Following is the query −

mysql> show create table demo66;

This will produce the following output −

+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 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)