Introduction to SQL Ordering and Dependencies

When working with relational databases, it’s common to have tables with interdependent data. In this article, we’ll explore how to sort rows relative to each other based on a foreign key (FK) relationship in SQL.

Understanding Foreign Keys and Their Implications

A foreign key is a field in a table that references the primary key of another table. This establishes a relationship between the two tables and ensures data consistency. In our case, we have a task table with an id, name, and dep column. The dep column forms a FK with the id column of the same table, meaning that each value in the dep column references a primary key in the id column.

The Problem: Sorting Based on Dependencies

We want to sort tasks such that a task will always appear after another task that it depends on. This requires us to determine the relative order of tasks based on their dependencies.

Using Recursive Common Table Expressions (CTEs)

The original poster’s attempt to solve this problem using standard SQL syntax was unsuccessful. However, as suggested by commenters, we can use recursive CTEs to establish a level for each task based on its relationship to its parent. This will allow us to sort tasks in the desired order.

Recursive CTE: A Step-by-Step Explanation

Let’s break down the recursive CTE example provided:

Creating the Test Data

CREATE TABLE `task` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `dep` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  CONSTRAINT `task_ibfk_1` FOREIGN KEY (`dep`) REFERENCES `task` (`id`)
);

INSERT INTO task (id, name, dep)
VALUES
  (1, 'A', NULL),
  (2, 'B', 4),
  (3, 'C', 2),
  (4, 'D', 1);

Recursive CTE: Establishing the Level

The recursive CTE consists of two parts:

Part 1: Base Case

WITH Test(id, name, dep) AS (
    SELECT 1, 'A', NULL UNION
    SELECT 2, 'B', 4 UNION
    SELECT 3, 'C', 2 UNION
    SELECT 4, 'D', 1 
)

In this part, we define the initial set of tasks with their dependencies. The UNION operator is used to combine multiple SELECT statements into a single CTE.

Part 2: Recursive Join

, RecursiveCTE AS (
  SELECT id, name, dep, 0 AS level
  FROM Test
  WHERE dep IS NULL
  UNION ALL
  SELECT i.id, i.name, i.dep, level + i.id
  FROM Test i
  INNER JOIN RecursiveCTE c ON c.id = i.dep
)

In this part, we recursively join the Test table with itself on the dep column. For each task, we calculate its level by adding the current id to the parent’s level.

Final Select and Order

SELECT id, name, dep
FROM RecursiveCTE
ORDER BY level;

Finally, we select all tasks from the recursive CTE and order them based on their calculated level.

Output and Explanation

The output of this query is:

id name dep
-- ---- ----
1  A    NULL
4  D    1
2  B    4
3  C    2

As we can see, the tasks are now sorted based on their dependencies:

• Task ‘A’ has no dependencies (level 0)
• Task ‘D’ depends on task ‘4’ (level 1)
• Task ‘B’ depends on task ‘4’ (level 4)
• Task ‘C’ depends on task ‘3’ which depends on task ‘2’, and therefore has a level of 2

Conclusion

In this article, we explored how to sort rows relative to each other based on foreign key relationships in SQL using recursive common table expressions. While the original poster’s approach was unsuccessful, the recursive CTE example provided demonstrates an effective solution for determining the relative order of tasks based on their dependencies.

Last modified on 2023-06-02