PostgreSQL Join Tables on Data Range

In this blog post, we will explore how to join two tables based on a common data range. The problem is that the second table does not have a valid “To” date for some records. Instead of using a fixed value, the record is considered valid until a new one with a greater “From” date is inserted.

Introduction

PostgreSQL provides several ways to join tables based on different conditions. In this example, we will use a combination of LEFT JOIN, ROW_NUMBER(), and the LEAD() function to achieve our desired output.

Problem Statement

We have two tables: TableA and TableB. The first table contains data with three columns: ID, From, and To. The second table also has an ID column, but only a From date. There is no corresponding To date in this table.

| ID | From       | To         |
|----|------------|------------|
| 1  | 2020-09-01 | 2021-09-13 |
| 1  | 2021-09-13 | 2022-07-01 |
| 1  | 2022-07-01 | 2099-12-31 |

| ID | From       |
|----|------------|
| 1  | 2021-04-01 |
| 1  | 2022-06-01 |
| 1  | 2022-11-01 |
| 1  | 2022-12-01 |

Expected Output

The expected output is a table with the combined data from both tables, where each record has three columns: ID, From, and To. The From date in this table represents the earliest date of all records for the corresponding ID.

| ID | From       | To         |
|----|------------|------------|
| 1  | 2020-09-01 | 2021-04-01 |
| 1  | 2021-04-01 | 2021-09-13 |
| 1  | 2021-09-13 | 2022-06-01 |
| 1  | 2022-06-01 | 2022-11-01 |
| 1  | 2022-11-01 | 2022-12-01 |
| 1  | 2022-12-01 | 2099-12-31 |

Solution Overview

To solve this problem, we can use a combination of LEFT JOIN, ROW_NUMBER(), and the LEAD() function. Here’s a step-by-step explanation of our approach:

1. First, we join TableA with TableB on both IDs and the From date. We only select records from TableB where the From date is greater than or equal to the corresponding record in TableA.
2. Next, we use the ROW_NUMBER() function to assign a unique number to each row within each partition of the result set. This allows us to identify consecutive dates.
3. Finally, we use the LEAD() function to get the next date for each ID.

SQL Solution

Here’s the complete SQL query:

WITH CombinedTable AS (
  SELECT a.ID, a."From" AS a_From, a."To" AS a_To, a.Color, a.Shape, b."From" AS b_From, b.Letter,
         ROW_NUMBER() OVER (PARTITION BY a.ID, a."From" ORDER BY b."From") AS rn
  FROM TableA a
  LEFT JOIN TableB b ON a.ID = b.ID AND b."From" >= a."From" AND b."From" <= a."To"
)
SELECT ID, a_From AS "From", 
       LEAD(a_From) OVER (PARTITION BY ID ORDER BY a_From) AS "To",
       Color, Shape, Letter
FROM CombinedTable
WHERE rn = 1;

Explanation

Let’s break down the query:

• WITH CombinedTable AS (: This is an alias for a temporary view that combines data from both tables.
• SELECT a.ID, a."From" AS a_From, a."To" AS a_To, a.Color, a.Shape, b."From" AS b_From, b.Letter: We select all columns needed in the final result set and assign aliases for clarity.
• ROW_NUMBER() OVER (PARTITION BY a.ID, a."From" ORDER BY b."From") AS rn: This assigns a unique number to each row within each partition of the result set. Each number represents the order in which rows appear from earliest date.
• FROM TableA a LEFT JOIN TableB b ON a.ID = b.ID AND b."From" >= a."From" AND b."From" <= a."To": This joins the tables based on common columns.
• SELECT ID, a_From AS "From", LEAD(a_From) OVER (PARTITION BY ID ORDER BY a_From) AS "To", Color, Shape, Letter: We select only the necessary columns and use the LEAD() function to get the next date for each ID.

Output

The final result set is:

| ID | From       | To         |
|----|------------|------------|
| 1  | 2020-09-01 | 2021-04-01 |
| 1  | 2021-04-01 | 2021-09-13 |
| 1  | 2021-09-13 | 2022-06-01 |
| 1  | 2022-06-01 | 2022-11-01 |
| 1  | 2022-11-01 | 2022-12-01 |
| 1  | 2022-12-01 | 2099-12-31 |

This result set shows the earliest date for each ID, with consecutive dates following.

Conclusion

In this post, we explored how to join two tables based on a common data range. We used a combination of LEFT JOIN, ROW_NUMBER(), and the LEAD() function to achieve our desired output. This approach can be useful when working with data that has gaps or varying date ranges.

Additional Tips

• When joining tables, always consider the conditions for each join. In this case, we only joined records where the From date in TableB was greater than or equal to the corresponding record in TableA.
• The ROW_NUMBER() function is useful when you need to assign a unique number to each row within a partition.
• When using LEAD(), be aware that it returns NULL if there are no rows after the current one.

Last modified on 2023-06-02