Talk about the principle of in query in mysql

MySQL is a relational database management system (RDBMS). In MySQL, IN query is a common query statement. It is used to retrieve the specified value from the database. The basic syntax of IN query is as follows:

SELECT * FROM table_name WHERE column_name IN (value1, value2, value3, ...);

This query will return specific columns (column_name) in all tables Rows whose value matches multiple values ​​specified (value1, value2, value3, etc.).

In MySQL, the IN query principle is implemented based on SET or TINYINT collection. A SET or TINYINT collection is used to represent all possible values ​​in a column. If the queried value matches any value in the collection, the row will be returned.

The SET collection is a column type containing enumerated columns. Each set can contain multiple values, each with a unique number. MySQL uses bit masks to represent collections. Each position represents the presence or absence of a value. If the value of this position is 1, it means that this value exists in the set, otherwise it does not exist. The result of the mask calculation is an integer. For example, if the value of the set is (1,5), the corresponding binary bit of the element in the set is 100001, and the calculation result is 33. When querying, when using the IN query statement, MySQL will convert the value of the data to be queried into a mask, and then query based on the mask.

The TINYINT collection is a smaller collection type than the SET collection and is only suitable for numbers between 0 and 255. TINYINT collections allow the creation of smaller, faster indexes.

One of the advantages of the IN query is that it can retrieve multiple values. Using multiple values ​​in a query instead of a single value can significantly reduce query time. In addition, IN queries can also be used with subqueries, which can simplify query codes and improve performance.

In general, this article introduces the principles of MySQL IN query in detail, including the mask calculation method and the advantages of IN query implemented using SET and TINYINT collections. Understanding these contents can help MySQL developers better understand IN queries and provide them with a reference for creating more efficient queries.

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