如何使一对列在MySQL中唯一？

要使一对列唯一，请使用带有ALTER TABLE命令的UNIQUE。以下是语法-

alter table yourTableName add unique yourUniqueName(yourColumnName1,yourColumnName2,...N);

让我们首先创建一个表-

create table DemoTable
(
   StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   StudentFirstName varchar(100),
   StudentLastName varchar(100),
   StudentAge int,
   StudentPhoneNumber varchar(20)
);

以下是在MySQL中创建一对唯一列的查询-

alter table DemoTable add unique DemoTable_unique_StudentFirstName_StudentPhoneNumber(StudentFirstName,StudentPhoneNumber);
Records: 0 Duplicates: 0 Warnings: 0

使用插入命令在表中插入一些记录-

insert into DemoTable(StudentFirstName,StudentLastName,StudentAge,StudentPhoneNumber) values('John','Doe',21,'9878567878');
insert into DemoTable(StudentFirstName,StudentLastName,StudentAge,StudentPhoneNumber) values('John','Smith',23,'7654674545');
insert into DemoTable(StudentFirstName,StudentLastName,StudentAge,StudentPhoneNumber)
values('John','Brown',21,'9878567878');
ERROR 1062 (23000): Duplicate entry 'John-9878567878' for key 'DemoTable_unique_StudentFirstName_StudentPhoneNumber'

使用select语句显示表中的所有记录-

select *from DemoTable;

这将产生以下输出-

+-----------+------------------+-----------------+------------+--------------------+
| StudentId | StudentFirstName | StudentLastName | StudentAge | StudentPhoneNumber |
+-----------+------------------+-----------------+------------+--------------------+
|         1 | John             | Doe             | 21         | 9878567878         |
|         2 | John             | Smith           | 23         | 7654674545         |
+-----------+------------------+-----------------+------------+--------------------+
2 rows in set (0.00 sec)