How to query and delete operations in MySQL database

MySQL database is a commonly used relational database management system. It has the advantages of high performance, high reliability and high scalability, and is one of the current mainstream databases. In the MySQL database, querying and deleting are very common operations. This article will introduce how to query and delete operations in the MySQL database.

1. Query operation

Query is one of the most basic operations in the MySQL database. Through query, the required data can be obtained, thereby providing the basis for business processing. In MySQL, the query statement is defined using the SELECT keyword, and its basic syntax structure is as follows:

SELECT column_name1,column_name2,...FROM table_name
WHERE condition;

Among them, column_name Indicates the column names required for the query, and you can use the wildcard * to represent all columns; table_name represents the table name to be queried; condition represents the query conditions, which can be combined using various operators and condition keywords.

The following are some common query statements:

1. Query all data:

SELECT * FROM table_name;

2. Query the data of the specified column ：

SELECT column_name1,column_name2,... FROM table_name;

3. Query data under specified conditions:

SELECT * FROM table_name WHERE condition;

4. Query and collect statistics:

SELECT COUNT(column_name) FROM table_name;

5. Query and sort data:

SELECT * FROM table_name ORDER BY column_name DESC /ASC;

6. Query and group data:

SELECT column_name, COUNT(column_name) FROM table_name GROUP BY column_name HAVING COUNT(column_name)>1;

above These are only some basic query statements. If you need to know more query statements, you can refer to the MySQL official documentation or related tutorials.

2. Delete operation

Delete operation is another common operation in MySQL database. Delete can clear useless data and free up storage space. In MySQL, the delete statement is defined using the DELETE keyword. Its basic syntax structure is as follows:

DELETE FROM table_name WHERE condition;

where table_name represents the name of the table to delete data; condition represents Deletion conditions can be combined using various operators and condition keywords.

The following are some common delete statements:

1. Delete all data:

DELETE FROM table_name;

2. Delete data under specified conditions ：

DELETE FROM table_name WHERE condition;

3. Limit the number of deleted data:

DELETE FROM table_name WHERE condition LIMIT num;

4.Delete All data in the table and retain the table structure:

TRUNCATE table_name;

The above are only some basic deletion statements. If you want to know more deletion statements, you can refer to the MySQL official documentation or related tutorials.

3. Query and delete operation

The query and delete operation is to find the data through query and then delete it, thereby avoiding the risk of accidentally deleting valid data. In MySQL, query and delete operations can be completed using subqueries. The sample code is as follows:

DELETE FROM table_name WHERE column_name IN (SELECT column_name FROM table_name WHERE condition);

Among them, column_name Represents the column name to be deleted, you can use * to represent all columns; table_name represents the name of the table to query and delete data; condition represents the query condition, which can be combined using various operators and condition keywords.

What you need to pay attention to when querying and deleting operations is that you must perform the query operation first to ensure that the data to be deleted meets the conditions before performing the deletion operation to avoid accidentally deleting valid data.

To sum up, the query and delete operation in the MySQL database is one of the basic data processing operations. Find the data to be deleted through query and then delete it to avoid the risk of accidentally deleting valid data. Query and delete operations need to select appropriate query statements and delete statements based on actual business needs, and you must query first and then delete to ensure that the deletion operation is correct.

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