MySQL database management deleting SQL statements

MySQL is an open source relational database management system commonly used for data processing and management of web applications. Deleting data is a common operation in database applications. MySQL uses the DELETE statement to delete data.

The DELETE statement is a type of SQL statement used to delete rows from one or more tables. As a general rule, deletion operations need to be done with caution to ensure that important data is not lost. Therefore, data should be backed up before deletion.

Syntax

DELETE FROM table name WHERE expression;

Among them, DELETE is the keyword of the delete operation, FROM specifies the table to delete, and WHERE specifies the table to be deleted. lines can be omitted if there is no need to limit the deleted lines.

Note when using:

1. When the DELETE statement is executed, the deleted data cannot be recovered.
2. When deleting a row of data, the data in the associated table will also be deleted.
3. If the WHERE expression is not specified, all data in the table will be deleted.

Example

Suppose we have a table named user, which contains the fields id, name and age. Now we want to delete the user information named "Tom". The SQL statement is as follows:

DELETE FROM user WHERE name='Tom';

After executing this statement, all names in the user table The row for "Tom" will be deleted.

MySQL also provides the LIMIT clause to specify the number of rows that need to be deleted. For example, we only need to delete the information of users under 7 years old, the statement is as follows:

DELETE FROM user WHERE age<7 LIMIT 10;

This statement deletes the information of users under 7 years old in the user table , and only delete up to 10 rows of data.

The DELETE command can not only delete row data, but also use the DROP statement to delete the entire table, including table definition, all data, indexes and other information. This is a very dangerous operation and should only be used when it is confirmed that the table is not needed.

Using the DROP statement to delete a table will clear the data of the entire table, so it must be used with caution. You can use the backup tool to back up the data first, and then perform the deletion operation.

Example

Suppose we want to delete the table named user. The SQL statement is as follows:

DROP TABLE user;

This statement will delete the entire user table and all data and definitions contained therein.

Summary

Delete operations are common operations in MySQL management and need to be used with caution. Before performing operations, you need to back up your data to ensure that important information will not be lost. The DELETE statement is used to delete data rows in the table. You can use the WHERE and LIMIT keywords to limit the deleted data rows. The DROP statement is used to delete the entire table and needs to be used with caution. MySQL is a feature-rich relational database management system that can easily perform data deletion operations using the DELETE statement.

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