sql 复合键

In a real-time situation, there can be scenarios when one column is not good enough to uniquely identify a row. In such scenarios, we use a combination of columns to uniquely identify a row.

在实时情况下，可能会出现一列不足以唯一标识一行的情况。 在这种情况下，我们使用列的组合来唯一地标识一行。

SQL复合键 (SQL Composite Key)

SQL composite key is a combination of two or more columns in a table that is used to uniquely identify a row. The combination of the columns guarantees uniqueness.

SQL复合键是表中两个或更多列的组合，用于唯一标识一行。 列的组合保证了唯一性。

We will try to understand how to create a composite key in the below-mentioned databases.

我们将尝试了解如何在下面提到的数据库中创建组合键。

1. MySQLMySQL
2. SQLServerSQL服务器
3. PostgreSQLPostgreSQL

MySQL复合键 (MySQL Composite Key)

Syntax: –

句法： -

CREATE TABLE table_name
(COL1 datatype,
COL2 datatype, COLn datatype
PRIMARY KEY (COL1, COL2));

In the syntax above, we can see that the primary key is a combination of two columns.

在上面的语法中，我们可以看到主键是两列的组合。

Let us create a table with a composite primary key.

让我们创建一个带有复合主键的表。

CREATE TABLE Employee(
employee_nointeger,
employee_name character(50),
employee_city character(35),
employee_phn numeric,PRIMARY KEY (employee_no,employee_phn));

In the table above, we are creating the primary key using two columns – employee_no and employee_phn.

在上表中，我们使用两列创建主键-employee_no和employee_phn。

Post-execution of the above query check the table property. The following screen will appear.

执行以上查询后，检查表属性。 将出现以下屏幕。

MySQL Composite Primary Key

MySQL复合主键

PostgreSQL复合密钥 (PostgreSQL Composite Key)

Syntax: –

句法： -

CREATE TABLE table_name
(COL1 datatype,
COL2 datatype, COLn datatype
PRIMARY KEY (COL1, COL2));

In the syntax above, we can see that the primary key is a combination of two columns.

在上面的语法中，我们可以看到主键是两列的组合。

Let us create a table with a composite primary key.

让我们创建一个带有复合主键的表。

CREATE TABLE Employee(
employee_no integer,
employee_name character(50),
employee_city character(35),
employee_phn numeric,PRIMARY KEY (employee_no,employee_phn));

In the table above, we are creating a primary key using two columns – employee_no and employee_phn.

在上表中，我们使用两列创建主键-employee_no和employee_phn。

Post-execution of the above query at PGAdmin, the following screen will appear.

在PGAdmin上执行上述查询后，将出现以下屏幕。

PostgreSQL Composite Primary Key

PostgreSQL复合主键

SQL Server复合键 (SQL Server Composite Key)

Syntax: –

句法： -

CREATE TABLE table_name
(COL1 datatype,
COL2 datatype, COLn datatype
PRIMARY KEY (COL1, COL2));

In the syntax above, we can see that the primary key is a combination of two columns.

在上面的语法中，我们可以看到主键是两列的组合。

Let us create a table with a composite primary key.

让我们创建一个带有复合主键的表。

CREATE TABLE Employee(
employee_no integer,
employee_name character(50),
employee_city character(35),
employee_phn numeric,PRIMARY KEY (employee_no,employee_phn));

Post-execution of the above query at SQL server. The following screen will appear.

在SQL Server上执行以上查询。 将出现以下屏幕。

SQLServer Composite Primary Key

SQLServer复合主键

摘要 (Summary)

SQL composite key is used only when a column is not suitable for uniquely identifying a row.

仅当列不适合唯一标识行时，才使用SQL复合键。

翻译自: https://www.journaldev.com/30173/sql-composite-key

sql 复合键