How is the with recursive clause used in MySQL?

In MySQL, the WITH RECURSIVE statement enables us to query and process data in a recursive manner. This feature is typically used for handling data with recursive structures, such as tree structures or hierarchical structures.

The basic syntax of the WITH RECURSIVE statement is as follows:

WITH RECURSIVE cte_name (column1, column2, ...) AS (
    -- 初始查询
    SELECT column1, column2, ...
    FROM table
    WHERE condition -- 初始查询条件
    
    UNION ALL
    
    -- 递归查询
    SELECT column1, column2, ...
    FROM table
    JOIN cte_name ON join_condition -- 递归查询条件
)
-- 主查询
SELECT column1, column2, ...
FROM cte_name;

The WITH RECURSIVE statement consists of three sections.

1. Initial query: Select initial data set from table using SELECT statement.
2. Recursive query: By using a SELECT statement to select additional datasets from a table that are associated with the initial dataset, and connecting them to the results of the recursive query using JOIN conditions. This process is repeated until the termination condition is met.
3. Main query: Select the desired fields from the final recursive dataset using the SELECT statement.

In recursive queries, we use the UNION ALL operator to combine the initial query results with the recursive query results. This way, each recursive query will merge the new result set with the previous result set, creating a larger result set each time.

It’s important to note that recursive queries must have a termination condition, otherwise it could result in an infinite loop. The termination condition can be defined using the conditions from the initial query or the recursive query.

By using the WITH RECURSIVE statement, we can easily handle data with recursive structures and perform operations such as traversing tree structures and calculating hierarchical structures.