mysql query foreign key

MySQL is a relational database management system, and one of its most basic functions is to support foreign keys. A foreign key is a method of joining two tables by defining a column in one table that is used to reference a primary key in the other table. Using foreign keys in MySQL can help us build a more powerful and flexible data model and improve efficiency when querying.

This article will introduce some issues and techniques about querying foreign keys in MySQL to help you better understand and use foreign keys in MySQL.

1. The basic concept of MySQL foreign keys

1. What is a foreign key?

Foreign key refers to a column in one table that refers to the primary key column of another table. This reference process establishes a relationship between the two tables, which is called referential integrity.

2. What is the role of foreign keys?

Through foreign keys, we can associate the data of one table with the data of another table, thereby achieving data diversification, sharing and updating. Through foreign keys, we can easily implement data query operations between tables and improve efficiency.

3. Type of foreign key?

In MySQL, foreign keys can be divided into the following two types:

(1) Integrity foreign key: a foreign key with integrity constraints, which defines a reference relationship, ensuring that references are only allowed if the referenced table exists.

(2) Ordinary foreign key: A foreign key without integrity constraints. It simply associates the data of one table with the data of another table.

2. How to query foreign keys in MySQL

1. Query foreign keys

Querying foreign keys is a very basic query operation in the MySQL database. Foreign keys can be queried in the following ways:

SELECT * FROM 表名 WHERE 外键列名 = 外键值;

Specifically, this statement finds records in the table that match the given value in the foreign key column and returns all information related to the record.

2. Connect related tables

When we need to find the data associated between two tables, we can use the join related table technology to connect the two tables together. Query operations. The specific usage is as follows:

SELECT * FROM 表1 JOIN 表2 ON 表1.外键列名 = 表2.主键列名 WHERE 外键列名 = 外键值;

The JOIN keyword in this statement represents the link between the two tables, and the ON keyword is used to specify the connection conditions between the tables. By connecting related tables, we can get more detailed and useful query results, and we can also more easily query data between multiple tables.

3. MySQL foreign key operations and restrictions

1. Adding foreign keys

It is very simple to add foreign keys in MySQL. Simply define a foreign key column with complete constraints in the table, and then use the following syntax to define the foreign key:

ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键列名) REFERENCES 参考表(主键列名);

It is worth noting that when adding a foreign key, you need to ensure that the referenced table already exists, otherwise it may Cause an error.

2. Deletion of foreign keys

It is also relatively easy to delete foreign keys in MySQL. You can use the following syntax to delete a foreign key:

ALTER TABLE 表名 DROP FOREIGN KEY 外键名;

When the foreign key is deleted, you need to ensure that the main table and reference table are modified, otherwise it may cause unexpected consequences.

3. Restrictions on foreign keys

In MySQL, there are some restrictions on foreign keys. For example:

(1) Two foreign keys with the same name cannot be defined in one table.

(2) Foreign keys cannot be used under the MYISAM engine of MySQL, but can only be used under the InnoDB engine.

(3) When modifying or deleting the primary key column of the referenced table, all related foreign keys of the table need to be deleted first.

In general, by learning the relevant content of foreign keys in MySQL, we can better use the foreign key function to diversify, share and update data, thereby improving the efficiency of data query operations. It should be noted that when using foreign key and related table technology, you need to pay attention to operational restrictions to ensure the integrity and validity of the data.

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