MySQL's Recursive CTE is mainly used to process hierarchical data. 1. It realizes traversal of the tree structure through initial queries and recursive queries, such as finding all subordinates of a certain employee; 2. It can be used to generate numerical sequences, such as 1 to 100; 3. When using them, be careful to avoid infinite loops, control recursive depth, optimize indexes, and first test small data to verify the correctness of the logic.
MySQL's Recursive CTE (recursive public table expression) is mainly used to process hierarchical data, such as organizational structure, classification trees, comment nesting, etc. Recursive CTE is very useful if you need to start from a node and find all its children or parents.
Here is a common example to illustrate how to use MySQL's Recursive CTE.
1. What is Recursive CTE?
CTE (Common Table Expression) is a temporary named result set that can be referenced in SQL queries. Recursive CTE is a special CTE that can repeatedly refer to itself, thereby realizing the traversal of hierarchical data.
In MySQL, the syntax structure of Recursive CTE is as follows:
WITH RECURSIVE cte_name AS (
-- Initial query (non-recursive part)
SELECT ...
UNION ALL
-- Recursive part SELECT ... FROM cte_name WHERE ...
)
SELECT * FROM cte_name;2. Example: Query the employees' subordinate teams
Suppose there is a employees table, the structure is as follows:
| id | name | manager_id |
|---|---|---|
| 1 | Alice | NULL |
| 2 | Bob | 1 |
| 3 | Carol | 2 |
| 4 | David | 2 |
| 5 | Eve | 1 |
Goal: Find all subordinates of Alice (id=1) (including both direct and indirect subordinates).
SQL queries are as follows:
WITH RECURSIVE subordinates AS (
-- Initial query: Find the SELECT id, name, manager_id of Alice
FROM employees
WHERE manager_id = 1
UNION ALL
-- Recursive query: Continue to find the subordinates' subordinates SELECT e.id, e.name, e.manager_id
FROM employees e
INNER JOIN subordinates s ON e.manager_id = s.id
)
SELECT * FROM subordinates;The execution results will include Bob, Carol, David, and Eve.
Note:
- The recursive termination condition is automatically terminated when no more records can be added.
-
manager_id = 1is just the starting point, you can change it to another ID according to your needs. - If the level is too deep, you may encounter performance problems or exceed the recursive depth limit (the default is 1000), which can be adjusted by setting
cte_max_recursion_depth.
3. Another common purpose: generate a sequence of numbers
Sometimes we need to generate a series of consecutive numbers, such as 1 to 100. Recursive CTE can also be used to do this.
WITH RECURSIVE numbers AS (
SELECT 1 AS n
UNION ALL
SELECT n 1 FROM numbers WHERE n < 100
)
SELECT * FROM numbers;Although this example is simple, it is very suitable for understanding recursive logic. You can also expand it to do more complex things, such as date sequences, time segment statistics, etc.
4. Several suggestions for using Recursive CTE
- Avoid infinite loops : Make sure that recursion has clear termination conditions, otherwise it will cause errors or query stuck.
- Control the depth of recursion : If the amount of data is large, appropriately limit the depth of recursion to avoid excessive resource consumption.
- Index optimization : Establishing indexes on recursive fields (such as parent_id, manager_id) can significantly improve performance.
- Test small data : First use a small amount of data to test whether the logic is correct, and then apply it to the full amount of data.
Basically that's all. Recursive CTE is very powerful when dealing with tree structures or hierarchical relationships. After mastering the basic structure, it can be flexibly applied to various scenarios.
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