Joining Multiple Tables with the Same Column Name

In this article, we will explore how to join multiple tables in SQL when they have the same column name. This is a common problem that arises when working with related data across different tables.

Understanding the Problem

The problem presents a scenario where we need to combine data from three tables: Table-1, Table-2, and Table-3. Each table has the same column names, specifically ‘Date’, ‘Brand’, and ‘Series’. However, there is no direct match between the columns of the three tables based on these common column names.

We are tasked with merging these tables into a single result set while maintaining the relationships between the original data. Specifically, we want to create a new table with the following structure:

If there is no match between the data in one of the tables, we want to fill in the missing values with zeros.

Solution Overview

To solve this problem, we can use a combination of SQL’s UNION ALL operator and aggregation functions. The key idea is to create a temporary table that combines the data from all three original tables using UNION ALL, and then apply aggregation functions to fill in the missing values.

Step 1: Creating the Temporary Table

We start by creating a temporary table that combines the data from the three original tables using UNION ALL. Each row in this temporary table will represent a single entry from one of the original tables, with additional columns to indicate which table each value comes from.

SELECT h.date, h.brand, h.series_name, h.amazone_viewer, 't1' as table_view
FROM `Table-1` h

UNION ALL

SELECT h1.date, h1.brand, h1.series, h1.viewes, 't2'
FROM `Table-2` h1

UNION ALL

-- Add the third table here...

Note that we’ve used a placeholder for the third table; we’ll fill in this section later.

Step 2: Filling in Missing Values

Next, we apply aggregation functions to each column of the temporary table. We use CASE statements to fill in missing values with zeros.

SELECT t.Date, t.Brand, t.Series_name,
       SUM(CASE WHEN t.table_view = 't1' THEN t.amazone_viewer ELSE 0 END) AS Table_1_Viewers,
       -- Add more aggregation functions here...
FROM (SELECT h.date, h.brand, h.series_name, h.amazone_viewer, 't1' as table_view
     FROM `Table-1` h UNION ALL
     SELECT h1.date, h1.brand, h1.series, h1.viewes, 't2'
     FROM `Table-2` h1 UNION ALL

     -- Add the third table here...
    ) t
GROUP BY t.Date, t.Brand, t.Series_name;

Step 3: Finalizing the Query

Finally, we add any additional aggregation functions to the query as needed. We’ll assume that there is one more column from Table-2 that we want to include in our final result set.

SELECT t.Date, t.Brand, t.Series_name,
       SUM(CASE WHEN t.table_view = 't1' THEN t.amazone_viewer ELSE 0 END) AS Table_1_Viewers,
       SUM(CASE WHEN t.table_view = 't2' THEN t.viewes ELSE 0 END) AS Table_2_Viewers,
       -- Add more columns here...
FROM (SELECT h.date, h.brand, h.series_name, h.amazone_viewer, 't1' as table_view
     FROM `Table-1` h UNION ALL
     SELECT h1.date, h1.brand, h1.series, h1.viewes, 't2'
     FROM `Table-2` h1 UNION ALL

     -- Add the third table here...
    ) t
GROUP BY t.Date, t.Brand, t.Series_name;

Conclusion

By using a combination of SQL’s UNION ALL operator and aggregation functions, we can effectively join multiple tables with the same column name while maintaining the relationships between the original data. This approach allows us to create a new table with all the desired columns and fill in missing values with zeros.

Note that this solution assumes that the three original tables have the same structure (i.e., the same column names). If the tables have different structures, we’ll need to modify the query accordingly.

Example Use Case

Suppose we have three tables: orders, products, and customers. Each table has a foreign key referencing the corresponding primary key in another table. We want to create a report that combines data from all three tables using UNION ALL and aggregation functions.

The final query might look like this:

SELECT o.order_id, p.product_name, c.customer_name,
       SUM(CASE WHEN o.order_type = 't1' THEN o.quantity ELSE 0 END) AS Order_1_Quantity,
       SUM(CASE WHEN o.order_type = 't2' THEN o.quantity ELSE 0 END) AS Order_2_Quantity
FROM (SELECT o.order_id, p.product_name, c.customer_name, 't1' as order_type
     FROM orders o JOIN products p ON o.product_id = p.id
     JOIN customers c ON o.customer_id = c.id UNION ALL

     SELECT o.order_id, p.product_name, c.customer_name, 't2'
     FROM orders o JOIN products p ON o.product_id = p.id
     JOIN customers c ON o.customer_id = c.id) t
GROUP BY t.order_id, t.product_name, t.customer_name;

This query combines data from the orders and products tables using UNION ALL, and joins it with the customers table using a foreign key. The result set includes all the desired columns, including aggregated quantities for each order type.

Note that this is just one possible solution to joining multiple tables with the same column name. Depending on your specific use case, you may need to modify the query accordingly.

Last modified on 2025-03-22