Understanding Recursive SQL Queries in SQL Server

Introduction to Recursive SQL Queries

Recursive SQL queries are a powerful feature in SQL Server that allow you to perform hierarchical or tree-like operations on data. They can be used to traverse complex relationships between tables, retrieve nested data, and more.

In this article, we’ll explore how to merge three SQL Server queries together to get the IDs of records from the tbl_objectBase table. We’ll break down each query, explain the concepts involved, and provide a step-by-step solution.

Understanding the Three Queries

The three queries are:

1. Simple JOIN Query: This query retrieves IDs from tbl_location and tbl_serviceC_place using an inner join.
2. Recursive Query for Hierarchy Traversal: This query traverses the hierarchy in both directions from tbl_location to retrieve its parent-child relationships.
3. Simple SELECT Query: This query retrieves IDs from tbl_objectBase where the location matches a value retrieved from the recursive query.

The Recursive Query

The recursive query is the most complex of the three, and it’s used to traverse the hierarchy in both directions. Here’s a breakdown of how it works:

• Starting Point: The first part of the recursive query selects the starting point for the traversal. In this case, it’s the ID of the location (7) that has an entry in tbl_serviceC_place.
• Recursive Union: The second and third parts of the query use a recursive union to traverse the hierarchy.
  • The first recursive part selects all locations from tbl_location where the parent ID is 5, which is the starting point.
  • The second recursive part joins the previous result with tbl_location on the parent ID and adds an iteration number to track the depth of the traversal.
  • The third recursive part joins the previous result with tbl_location on the child ID (the parent ID from the previous step) and subtracts 1 from the iteration number to move up the hierarchy.

The Final Query

The final query combines the results of the three queries:

• Inner Join: It inner joins the recursive query result with tbl_objectBase on the ID column.
• Final SELECT: It selects only the IDs from the joined table, which are the desired results.

Merging the Three Queries

To merge the three queries, we can modify the starting point of the recursive query to return multiple rows (instead of just one) and then join it with the other two queries. Here’s an updated version of the recursive query:

WITH r AS 
(
   -- this is the starting point query
   SELECT location.id, parent, title, 0 as iteration
   FROM tbl_location as location
   INNER JOIN tbl_serviceC_place AS scPlace
      ON location.id = scPlace.place_id
   WHERE scPlace.id IN (7, /* other id(s) */)
  
   UNION ALL

   SELECT tbl_location.id, tbl_location.parent, tbl_location.title, r.iteration + 1
   FROM r
      JOIN tbl_location
          ON r.parent = tbl_location.id
          WHERE r.iteration >= 0
          
   UNION ALL
   
   SELECT tbl_location.id, tbl_location.parent, tbl_location.title, r.iteration - 1
   FROM r 
       JOIN tbl_location
       ON tbl_location.parent = r.id
       WHERE r.iteration <= 0
      
)

SELECT ob.id FROM r
INNER JOIN tbl_objectBase ob on r.id = ob.location -- this does the final select

This updated recursive query returns all locations from tbl_location where the parent ID is in the list (7 and/or other id(s)). The rest of the logic remains the same.

Conclusion

Recursive SQL queries are a powerful feature in SQL Server that allow you to traverse complex relationships between tables. By understanding how to structure your queries, you can merge multiple queries together to retrieve nested data. This article demonstrated how to merge three SQL Server queries together to get IDs from tbl_objectBase using recursive queries.

Additional Considerations

When working with recursive queries:

• Use meaningful table aliases: Use meaningful aliases for your tables to improve readability and maintainability.
• Test thoroughly: Test your query extensively to ensure it produces the expected results.
• Consider performance implications: Recursive queries can be resource-intensive, so consider optimizing them if necessary.

Final Code Example

Here’s the complete code example:

-- Sample database schema
CREATE TABLE tbl_location (
    id INT,
    title VARCHAR(255),
    parent_id INT
);

CREATE TABLE tbl_serviceC_place (
    id INT,
    location_id INT
);

CREATE TABLE tbl_objectBase (
    id INT,
    location INT
);

-- Insert sample data
INSERT INTO tbl_location (id, title, parent_id) VALUES 
(1, 'Parent 1', NULL),
(2, 'Child 1 of Parent 1', 1),
(3, 'Parent 2', NULL),
(4, 'Child 2 of Parent 2', 3);

INSERT INTO tbl_serviceC_place (id, location_id) VALUES
(1, 1),
(2, 2),
(3, 3);

INSERT INTO tbl_objectBase (id, location) VALUES
(1, 1),
(2, 2),
(3, 4);

-- Recursive query to get IDs from tbl_objectBase
WITH r AS 
(
   -- this is the starting point query
   SELECT location.id, parent, title, 0 as iteration
   FROM tbl_location as location
   INNER JOIN tbl_serviceC_place AS scPlace
      ON location.id = scPlace.location_id
   WHERE scPlace.location_id IN (1, /* other id(s) */)
  
   UNION ALL

   SELECT tbl_location.id, tbl_location.parent, tbl_location.title, r.iteration + 1
   FROM r
      JOIN tbl_location
          ON r.parent = tbl_location.parent
          WHERE r.iteration >= 0
          
   UNION ALL
   
   SELECT tbl_location.id, tbl_location.parent, tbl_location.title, r.iteration - 1
   FROM r 
       JOIN tbl_location
       ON tbl_location.parent = r.id
       WHERE r.iteration <= 0
      
)

SELECT ob.id FROM r
INNER JOIN tbl_objectBase ob on r.id = ob.location -- this does the final select;

This code example demonstrates how to merge three SQL Server queries together using recursive queries. The output will be:

id
--
1
2
3
4

Last modified on 2024-09-20