Setting methods and precautions for MySQL composite primary key

MySQL Composite Primary Key Setting Method and Precautions

In the MySQL database, the primary key is a field or combination of fields used to uniquely identify each record in the table . In addition to setting a single field as a primary key, you can also set a combination of multiple fields as a composite primary key. This article will introduce the setting method, usage scenarios and precautions of composite primary keys in MySQL, and attach specific code examples.

How to set a composite primary key:
When creating a table, you can set a composite primary key through the following syntax:

CREATE TABLE table_name (
    column1 data_type,
    column2 data_type,
    ...
    PRIMARY KEY (column1, column2)
);

column1 and column2 are two fields in the table, which together form the composite primary key. When creating a table, enclose these two fields in parentheses and add the PRIMARY KEY keyword before the parentheses to set them as composite primary keys.

Scenarios for using composite primary keys:

1. When each record in a table needs to be uniquely identified by multiple fields, it is suitable to use composite primary keys. For example, in an order table, the order number and customer number together can uniquely determine an order record.
2. When performing data query and sorting, if you need to operate based on multiple fields, using composite primary keys can improve query performance.

Note:

1. The field order of the composite primary key is important, because the order of the fields determines the order of the index. When designing a composite primary key, the order of fields needs to be determined based on specific business requirements.
2. For composite primary key fields, you should try to choose immutable fields as the primary key to avoid frequently modifying field values ​​on the primary key to avoid affecting database performance.
3. The composite primary key must ensure uniqueness when inserting data, otherwise the insertion will fail. You can use the INSERT IGNORE or ON DUPLICATE KEY UPDATE statement to handle duplicate primary key situations.

The following is a specific example:

Suppose there is a table named student, which contains the student's student number (student_id), class number (class_id ) and name fields, now you need to combine the two fields student_id and class_id as a composite primary key.

CREATE TABLE student (
    student_id INT,
    class_id INT,
    name VARCHAR(50),
    PRIMARY KEY (student_id, class_id)
);

Through the above example, we can see how to set up a composite primary key in MySQL, and understand the usage scenarios and precautions for composite primary keys. In practical applications, according to specific business needs and database design specifications, reasonable setting of composite primary keys can improve database performance and data integrity.

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