Joins with two tables conditionally based on the value of ONE column

Introduction

In this blog post, we will explore how to perform a conditional join between three tables: purchase, item, and either supplier or officer. The goal is to retrieve data from these tables in a way that depends on the value of a specific column. We’ll use a combination of SQL joins and case statements to achieve this.

Background

The problem presented involves two tables, purchase and item, with a common column item_id. Each row in these tables corresponds to a purchase or an item, respectively. Additionally, we have another table, either supplier or officer, which contains information about the suppliers or officers involved in each purchase.

The goal is to join these three tables based on the value of the type column in the purchase table. This type can be either ‘purchase’ or ‘issue’. When the type is ‘purchase’, we want to retrieve the corresponding supplier name from the supplier table; when it’s ‘issue’, we want to retrieve the corresponding officer name from the officer table.

Problem Analysis

The initial query provided attempts to solve this problem by joining the purchase table with the item table and then filtering the results based on the sup_id column in the supplier table. However, this approach does not consider the conditional join between the purchase table and either the supplier or officer tables.

Solution Overview

To solve this problem, we can use a combination of SQL joins (both inner and left joins) and case statements to determine which column value to retrieve from the supplier or officer tables. The solution involves:

Joining the purchase table with both the item table and either the supplier or officer table using a left join.
Using a case statement to conditionally select the desired column value based on the type column in the purchase table.

Step-by-Step Solution

Query 1: Incorrect Initial Approach

The initial query attempts to solve this problem by joining the purchase table with the item table and then filtering the results based on the sup_id column in the supplier table.

SELECT item.item_name, 
       supplier.supplier_name, 
       purchase.date, 
       purchase.qty, 
       purchase.type 
FROM purchase 
JOIN item ON item.item_id = purchase.item_id 
WHERE supplier.sup_id IN (SELECT sup_id FROM supplier)

However, this query does not consider the conditional join between the purchase table and either the supplier or officer tables.

Query 2: Corrected Approach with Join

To solve this problem correctly, we need to perform a left join between the purchase table and both the item table and either the supplier or officer tables.

SELECT item.item_name, 
       CASE purchase.type 
         WHEN 'purchase' THEN supplier.supplier_name 
         WHEN 'issue' THEN officer.officer_name 
       END AS `supplier_name_officer_name`
FROM purchase 
JOIN item ON item.item_id = purchase.item_id 
LEFT JOIN supplier ON purchase.sup_id = supplier.sup_id
LEFT JOIN officer ON purchase.sup_id = officer.officer_id

In this corrected query, we use a left join to combine the three tables. The case statement then determines which column value to retrieve based on the type column in the purchase table.

Discussion

The corrected query demonstrates how to perform a conditional join between three tables using SQL joins and case statements. By using a combination of inner and left joins, we can effectively retrieve data from these tables while depending on the value of a specific column.

Conclusion

In this blog post, we explored how to perform a conditional join between three tables based on the value of a specific column. We analyzed the problem and presented a corrected approach that uses SQL joins and case statements to achieve the desired result. This solution can be applied in various real-world scenarios where data needs to be retrieved from multiple tables depending on certain conditions.

Last modified on 2024-09-17