[Mysql Series] Several methods to delete data in mysql

write in front 

Deleting data is a common operation in a MySQL database, which allows the removal of data that is no longer needed from a table. When performing deletion operations, you need to be careful to avoid accidentally deleting important data.

Method introduction

Here are several ways to delete data in MySQL:

1. DELETE statement
2. DROP TABLE statement
3. TRUNCATE TABLE
4. Use foreign key constraints

1. DELETE statement

The DELETE statement is one of the most commonly used methods to delete data. It allows you to delete data from a table based on specific conditions. The following is the basic syntax of the DELETE statement:

DELETE FROM table_name WHERE condition;

• table_nameIs the name of the table where data is to be deleted.
• conditionis an optional condition that specifies the rows to be deleted. If no condition is provided, all data in the table will be deleted.

Example:  

Assuming there is a studentstable named, you can use the following DELETE statement to delete records of students who are younger than 18 years old:

DELETE FROM students WHERE age < 18;

2. DROP TABLE statement:

The DROP TABLE statement is used to delete the entire table, including the table structure and data. This is a dangerous operation and needs to be used with caution because once performed, the data cannot be recovered.

DROP TABLE table_name;

Example:

If you want to completely delete productsthe table named and all its data, you can use the following DROP TABLE statement:

DROP TABLE products;

3. TRUNCATE TABLE

TRUNCATE TABLE is an operation in MySQL used to quickly delete all data in a table . Unlike the DELETE statement, the TRUNCATE TABLE operation not only deletes the data in the table, but also leaves the table's structure intact .

This means that the definitions of the table's columns, indexes, primary keys, etc. will not be affected, but the data will be cleared.

The following is a detailed introduction to the TRUNCATE TABLE operation:

grammar:

TRUNCATE TABLE table_name;

• table_nameIs the name of the target table to perform the TRUNCATE operation.

Features and Notes:

1. Fast: TRUNCATE TABLE is usually much faster than using the DELETE statement to delete all data. This is because TRUNCATE does not delete data row by row, but directly deletes data pages from the table, so it is more efficient.

2. Cannot use WHERE clause: TRUNCATE TABLE does not allow the use of WHERE clause to specify specific deletion conditions. It always deletes all data in the entire table. If you need to delete data conditionally, you should use the DELETE statement.

3. Delete operations are not logged: TRUNCATE TABLE operations generally do not generate transaction logs, so delete operations are not logged. This saves disk space and improves performance. However, this also means that deleted data cannot be recovered by rolling back. If you need to record a delete operation and be able to roll it back, you should use the DELETE statement and execute it within a transaction.

4. Automatically reset AUTO_INCREMENT counter: When data is deleted using TRUNCATE TABLE, the AUTO_INCREMENT counter associated with the table is automatically reset to 1. This means that the next time new data is inserted, counting will start from 1.

5. Appropriate permissions required: Sufficient permissions are required to perform the TRUNCATE TABLE operation. Normally, only users with DELETE permission on the table can perform TRUNCATE TABLE operations.

Example:

Assuming there is a employeestable named, you can use the TRUNCATE TABLE operation to clear all employee data from the table but preserve the structure of the table:

TRUNCATE TABLE employees;

This will quickly empty employeesthe table, but the structure, indexes, and other definitions of the table will remain unchanged and new data can be reinserted immediately.

In summary, TRUNCATE TABLE is a useful tool for quickly deleting all data in a table, especially when you need to empty the table without affecting the table structure .

However, since it does not support conditional deletion and does not record operation logs, you should choose whether to use TRUNCATE TABLE or DELETE statements to perform delete operations based on specific needs.

4. Use foreign key constraints:

If there is a foreign key relationship between tables, you can use foreign key constraints to delete data. Foreign key constraints ensure that deletion operations do not violate referential integrity.

Delete operations will automatically cascade to data in related tables.

Example:  

Suppose there are two tables ordersand order_items, order_itemstables have ordersforeign keys pointing to tables.

You can use the DELETE statement to delete ordersorders in the table, and the foreign key constraints will automatically delete order_itemsthe corresponding order items in the table.

DELETE FROM orders WHERE order_id = 123;

Method advantages and disadvantages

The following is a summary of the pros and cons of the previously mentioned methods of deleting data in MySQL:

1. DELETE statement:

• advantage:
  • Allows data to be deleted based on conditions, providing flexibility.
  • Can record deletion operations and support transactions.
  • The AUTO_INCREMENT counter will not be reset and the table structure will be preserved.
• shortcoming:
  • Slower when deleting large amounts of data because it deletes row by row.
  • Transaction logs will be generated, which may increase the load on the database.

2. TRUNCATE TABLE statement:

• advantage:
  • Very fast, especially suitable for clearing data of large tables.
  • No transaction log is generated, reducing the burden on the database.
  • The table structure is preserved and the AUTO_INCREMENT counter is not reset.
• shortcoming:
  • Conditional deletion cannot be specified, and the data in the entire table is always deleted.
  • Deletion operations are not recorded and cannot be rolled back.
  • Appropriate permissions are required.

3. DROP TABLE statement:

• advantage:
  • Quickly delete an entire table and its data.
  • No transaction log will be generated, reducing the burden on the database.
• shortcoming:
  • Completely delete the table, including table structure and data, and cannot be recovered.
  • It needs to be used with caution as it can easily lead to data loss.

4. Use foreign key constraints:

• advantage:
  • Used to maintain data integrity and ensure that deletion operations will not destroy reference relationships.
  • Automatic cascade deletion of related data.
• shortcoming:
  • Foreign key relationships need to be defined in the table design.
  • Cannot be used to quickly clear an entire table or delete large amounts of data.

Summarize 

These are the main methods of deleting data in MySQL. When performing deletion operations, make sure to back up important data to avoid accidental deletion. Additionally, choose the appropriate deletion method based on your needs to ensure database integrity and performance. If you are not sure how to perform the deletion, it is recommended to test it in a test environment to avoid surprises in the production environment.