How to use mysql cascade delete function

1. Review of MySQL basic knowledge

Before performing the MySQL cascade delete operation, we need to understand some basic knowledge of MySQL.

1. Characteristics of relational database

Relational database refers to a database that uses tables to organize data. Its characteristics are:

(1) Data is stored in table form, and each table represents an entity type.

(2) The columns in the table represent different attributes, and the rows represent different instances.

(3) Tables can be connected through primary key-foreign key relationships.

2. Keywords

There are many keywords in MySQL, such as: SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY, JOIN, etc. These keywords are used to perform different database operations.

3. Basic operations

MySQL has four basic operations: SELECT, INSERT, UPDATE and DELETE. Among them, DELETE is used to delete data in the database, which is the knowledge point we are going to talk about today - cascade deletion.

2. Basic knowledge of MySQL cascade delete

In MySQL, cascade delete mainly involves the issue of foreign key constraints. Foreign keys are used in databases to establish relationships between tables. In order to maintain the integrity and consistency of the data, when there are foreign key constraints, the related slave table records need to be deleted first, and then a record in the master table can be deleted.

1. Foreign key constraints

In a relational database, foreign key constraints are used to maintain associations between tables. In short, two or more tables are connected through foreign keys.

In MySQL, foreign key constraints are mainly divided into two categories: cascade control and restriction control. Cascading control can perform cascading operations on the row where the foreign key is located when performing operations, including cascading updates and cascading deletes.

2. Cascade deletion

In MySQL, when using foreign key constraints, you can set cascade deletion rules to automatically delete slave table records when deleting master table records. It is actually a statement "ON DELETE CASCADE", which will delete all related records from the table to maintain data consistency.

For example, suppose we have two tables: posts and comments. Among them, the posts table has an ID as the primary key, and the comments table has a post_id as a foreign key, which is used to establish an association with the posts table. It looks like this:

posts Form:

ID	Title
1	Article 1
2	Article 2
3	Article 3

comments form:

##IDpost_idComment content11This is the comment of the first article22This is the comment of the second article32This is the second comment on the second article43This is the comment on the third articleIf we want to delete the record of article 1, we also need to delete all the comments associated with it. We can write a statement like this:

DELETE FROM posts WHERE ID=1;

This command will delete the record with ID 1 in the posts table, and because we have set the cascade deletion rule, all records in the comments table associated with it will also be deleted. will be deleted, and the final result is:

posts Form:

IDTitle2Article 2##3comments form:

Article 3

ID234

3. Restrictions and precautions for cascade deletion

Although cascade deletion is very convenient in database operations, there are still some restrictions and precautions that need to be observed. We should pay special attention when using cascade delete to avoid unexpected situations during the operation.

1. Cascade deletion can only be performed through foreign keys

When using the cascade delete function, you need to set foreign key constraints first. If the foreign key constraint is not set, cascading delete cannot be used and the command will not be executed.

2. The deleted records cannot be records associated with other tables

If a record is associated with multiple tables, it cannot be deleted directly. We need to remove references to the record from other tables before deleting the record. Otherwise, cascade deletion will fail or lead to some unpredictable consequences.

3. Cascade deletion may cause data inconsistency

Although cascade deletion is very convenient for us, this function may cause data inconsistency during execution, especially When dealing with large amounts of data, you must proceed with great caution.

4. The slave table records must be deleted first and then the master table records

When using cascade delete, the slave table records must be deleted first and then the master table records. Otherwise, the delete operation will fail.

5. Cascade deletion may affect the correctness of the program

Automatically performing deletion operations may affect the correctness of the program, especially when deleting important data in the database. Therefore, we must carefully think about and predict the possible consequences of the cascading delete function.

post_id	Comment content
2	This is the comment of the second article
2	This is the second comment on the second article
3	This is the comment on the third article

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