How Can I Merge Two SQL Tables with Overlapping Dates and Ensure Unique Date Values in the Result?

Merging Two Tables with Unique Date Values in SQL

In data integration scenarios, merging tables with similar structures but potentially overlapping data can pose a challenge, especially when handling duplicate values. Consider the following two tables:

CREATE TABLE Inbound (
    Inbound_Date DATE,
    Product TEXT,
    InboundType TEXT,
    Quantity VARCHAR(255)
);

CREATE TABLE Outbound (
    Outbound_Date DATE,
    Product TEXT,
    OutboundType TEXT
);

Using a traditional UNION query, these tables can be merged as follows:

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;

However, this approach allows duplicate dates to exist. To ensure that each date is unique, we can modify the query to use UNION ALL and GROUP BY:

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;

This query first performs a UNION ALL operation to combine the rows from both tables, creating a single set of rows with overlapping dates and products. It then uses a nested query block to calculate the sum of inbound and outbound quantities for each date and product combination. Finally, the GROUP BY clause is applied to eliminate duplicate date values, ensuring that each date appears only once in the final result.

As a result, the modified query produces a table with unique dates, which can be useful for further analysis or data manipulation tasks.

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