mysql query foreign keys

In the MySQL database, a foreign key means that one or more columns in one table can refer to columns in another table, forming a relationship between the master table and the slave table. Foreign keys are one of the important means to ensure data integrity and consistency. But in actual development, we often need to query data associated with foreign keys, and query operations involving foreign keys need to pay attention to some issues.

This article will introduce in detail the relevant knowledge of MySQL query foreign keys.

1. Introduction to foreign keys

Foreign key, also known as external key and associated key, is a constraint used in MySQL database to maintain the integrity and consistency of data. It relates a column of one table to a column of another table.

When a table has a foreign key relationship with another table, they form the relationship between the master table and the slave table. The columns of the master table are called primary keys, and the columns of the slave tables are called foreign keys.

Foreign key constraints can be divided into two categories:

1. Cascade update (CASCADE UPDATE): When the primary key of the primary table is updated, the foreign key of the secondary table will be updated accordingly.

2. Cascade delete (CASCADE DELETE): When the primary key of the primary table is deleted, the foreign key of the secondary table will also be deleted accordingly.

2. Foreign key query

1. Query a single foreign key

When we need to query the data associated with a foreign key in a column of a table, we can use INNER JOIN performs related queries. The specific operations are as follows:

SELECT 表1.列1, 表2.列2 
FROM 表1 
INNER JOIN 表2 
ON 表1.外键列 = 表2.主键列

Among them, table 1 and table 2 are the tables that need to be associated, column 1 is the column to be queried in table 1, column 2 is the column to be queried in table 2, and the foreign key column is the column in table 1 that is associated with table 2, and the primary key column is the primary key column in table 2.

Example:

We have two tables, one is the student table and the other is the grades table. The student table has a student number field, and the grades table has a foreign key field related to the student number field in the student table. We need to query the names, subjects and grades of students in the grade table. The specific operations are as follows:

SELECT s.name, c.subject, c.score 
FROM student s 
INNER JOIN score c 
ON s.sid = c.sid

2. Query multiple foreign keys

When we need to query multiple foreign key relationships in a table When collecting data, we can use multiple INNER JOINs to perform related queries. The specific operations are as follows:

SELECT 表1.列1, 表2.列2, 表3.列3, …
FROM 表1
INNER JOIN 表2 
ON 表1.外键列1 = 表2.主键列1
INNER JOIN 表3 
ON 表2.外键列2 = 表3.主键列2
…

Among them, Table 1, Table 2, and Table 3 are the tables that need to be associated, Column 1 is the column to be queried in Table 1, and Column 2 is the column to be queried in Table 2. Column 3 is the column to be queried in table 3, foreign key column 1 is the column in table 1 that is associated with table 2, foreign key column 2 is the column in table 2 that is associated with table 3, and primary key column 1 is the column in table 2 The primary key column of , primary key column 2 is the primary key column in table 3.

Example:

We have three tables, one is the student table, one is the subject table, and one is the grade table. The student table and subjects table each have an id field. There are two foreign key fields, student id and subject id, in the grade table that relate to the id fields in the student table and subjects table. Now we need to query the names, subjects and grades of students in the grade table. The specific operations are as follows:

SELECT s.name, se.subject, c.score 
FROM student s 
INNER JOIN score c 
ON s.sid = c.sid 
INNER JOIN subject se
ON c.subject_id = se.id

3. Precautions for querying foreign keys

When using foreign keys to query, you need to pay attention to the following Several aspects:

1. Make sure that the related table already exists.

Before establishing a foreign key relationship, you need to ensure that both tables already exist. Otherwise, an error will occur when creating the foreign key relationship.

2. Ensure that the data type and length of the foreign key column and the primary key column are consistent.

When establishing a foreign key association, you need to ensure that the data type and length of the associated foreign key column and the primary key column are consistent, otherwise an error will occur. At the same time, you also need to pay attention when making queries, otherwise there may be problems with the query results.

3. Avoid NULL values ​​in foreign key columns.

When performing foreign key queries, avoid NULL values ​​in foreign key columns. The presence of NULL values ​​will cause inconsistent query results.

4. Avoid large data joins.

When performing multi-table queries, be careful to avoid large amounts of data joins, otherwise query performance will be seriously affected.

5. Pay attention to the cascading operations of foreign key relationships.

When querying using foreign key relationships, you need to pay attention to the cascading operations of foreign key constraints to avoid delete or update operations from causing adverse effects on the associated table.

4. Summary

Through the introduction of this article, I believe that everyone has understood the basic concepts and operation methods of MySQL foreign key query, as well as the issues that need to be paid attention to when performing foreign key query. In actual development, it is necessary to reasonably apply foreign key relationships for data query based on specific business needs and database table structure design, which can not only improve query efficiency, but also ensure data integrity and consistency.

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