Merging Two Tables in One SQL Query and Making Date Values Unique Using GROUP BY and UNION

Merging Two Tables in One SQL Query and Making Date Values Unique

In this article, we will explore how to merge two tables into one SQL query and make the date values unique. We will start with a basic explanation of SQL queries and then dive into the specifics of merging tables.

Introduction to SQL Queries

A SQL (Structured Query Language) query is a request made by an application or user to access, modify, or manage data in a database. SQL queries can be used to perform various operations such as creating new records, updating existing records, deleting records, and retrieving specific records.

SQL queries are composed of several components:

SELECT Statement: Used to retrieve specific records from the database.
FROM Clause: Specifies the table(s) from which data is retrieved.
WHERE Clause: Used to filter records based on certain conditions.
GROUP BY Clause: Groups rows that contain the same values in a specified column or columns.

Merging Tables

Merging two tables into one SQL query can be achieved using the UNION operator. The UNION operator combines the result sets of two or more SELECT statements and returns them as a single result set.

However, when working with dates, it is essential to consider how to handle duplicate dates. By default, SQL databases use the date in its stored format, which can sometimes lead to unexpected results.

Problem Statement

We are given two tables: Inbound and Outbound. The first table contains information about products that have arrived at a warehouse, while the second table contains information about products that have left the warehouse. We want to merge these two tables into one query and make the date values unique.

The Current Query

SELECT 
    Inbound_Date As Date, 
    Product, 
    SUM(Quantity) as Inbound, 0 as Outbound
FROM Inbound
GROUP BY 1,2

UNION ALL

SELECT 
    Outbound_Date,
    Product,
    0 as Inbound, COUNT("Outbound_Type")  as Outbound 
FROM Outbound
GROUP BY 1,2

ORDER BY 1,2;

This query works perfectly for producing the desired output. However, when we try to make the date values unique by modifying the query, we encounter issues.

Modifying the Query

To modify the query and make the date values unique, we need to consider how to handle duplicate dates. The current query uses the UNION ALL operator, which combines the result sets of two or more SELECT statements and returns them as a single result set. However, when working with dates, it is essential to consider how to handle duplicate dates.

One way to achieve this is by using the GROUP BY clause along with the UNION operator. The GROUP BY clause groups rows that contain the same values in a specified column or columns.

Modified Query

SELECT Date, Product, SUM(Inbound) as Inbound, SUM(Outbound) as Outbound
FROM ((SELECT Inbound_Date As Date, Product, SUM(Quantity) as Inbound, 0 as Outbound
      FROM Inbound
      GROUP BY 1,2
     ) UNION ALL
     (SELECT Outbound_Date,
            Product,
            0 as Inbound, COUNT(*)  as Outbound 
            FROM Outbound
            GROUP BY 1,2
           )
    ) io
GROUP BY Date, Product;

In this modified query, we use the UNION operator to combine the result sets of two or more SELECT statements. We then use the GROUP BY clause to group rows that contain the same values in the Date and Product columns.

Explanation

The key difference between the original query and the modified query is the addition of the GROUP BY clause. The GROUP BY clause groups rows that contain the same values in a specified column or columns, which helps to ensure that duplicate dates are handled correctly.

In the first SELECT statement, we group by Inbound_Date As Date, Product. This means that all records with the same date and product will be combined into one row. In the second SELECT statement, we group by Outbound_Date, Product. This means that all records with the same date and product will be combined into one row.

The UNION operator combines these two result sets and returns them as a single result set.

Conclusion

In this article, we explored how to merge two tables in one SQL query and make the date values unique. We discussed the basics of SQL queries and then dove into the specifics of merging tables. We also examined the original query and modified it to use the GROUP BY clause along with the UNION operator.

Last modified on 2024-05-04