MySQL is a popular relational database management system. Its powerful functions and flexibility make it the first choice for many enterprises and developers. Among them, MySQL's cascade delete function is also a feature that has attracted much attention.
The so-called cascade deletion is an operation that when deleting a record in the main table, related records in the associated slave table are also deleted. This function is very convenient and can avoid the trouble of frequently manually deleting related data.
However, cascade deletion also has certain risks and limitations. He needs to use it carefully and comply with MySQL's set of rules. This article will introduce the usage and precautions of MySQL cascade delete. I hope it will be helpful to everyone.
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
There are four basic operations in MySQL: 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. Under foreign key constraints, if you want to delete a record in the master table, you must first delete the related records in the slave table to ensure the integrity and consistency of the data.
1. Foreign key constraints
Foreign key constraints are a mechanism in a relational database, which is used to maintain the association 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. Among them, cascading control allows cascading operations on the row where the foreign key is located when performing certain operations, including cascade updates and cascade 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:
post_id | Comment content | |
---|---|---|
1 | This is the comment of the first article | |
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 |
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:
Title | |
---|---|
Article 2 | |
Article 3 |
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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