How Can We Combine KnownHours and UnknownHours Tables for a Unified Hours Report?

Combining Tables for a Unified Output

Introduction:
When dealing with multiple tables, it becomes necessary to merge them in order to provide a comprehensive and meaningful output. Combining tables involves techniques such as joining, union, and aggregation.

Problem Statement:
The task at hand is to combine two tables, KnownHours and UnknownHours, into a single output that groups hours by ChargeNum and CategoryID, ignoring Month.

Table Details:

Table	Column	Description
KnownHours	ChargeNum	Unique charge number
KnownHours	CategoryID	Category associated with the charge
KnownHours	Month	Month when the charge was incurred
KnownHours	Hours	Known hours for the charge
UnknownHours	ChargeNum	Unique charge number
UnknownHours	Month	Month when the charge was incurred
UnknownHours	Hours	Unknown hours for the charge

Desired Output:
The expected output is a single table with the following structure:

Table	Column	Description
Consolidated	ChargeNum	Unique charge number
Consolidated	CategoryID	Category associated with the charge or 'Unknown'
Consolidated	Hours	Total hours for the charge

Solution:
To achieve the desired output, we will utilize the UNION operation in SQL. UNION combines the results of multiple queries into a single table. In this case, we will run two queries, one for the KnownHours table and one for the UnknownHours table. The queries will aggregate the hours for each ChargeNum and CategoryID (and 'Unknown' for UnknownHours).

The first query for the KnownHours table:

SELECT ChargeNum, CategoryID, SUM(Hours)
FROM KnownHours
GROUP BY ChargeNum, CategoryID

The second query for the UnknownHours table:

SELECT ChargeNum, 'Unknown' AS CategoryID, SUM(Hours)
FROM UnknownHours
GROUP BY ChargeNum

Finally, we will combine the results of these two queries using UNION ALL:

SELECT ChargeNum, CategoryID, SUM(Hours)
FROM KnownHours
GROUP BY ChargeNum, CategoryID
UNION ALL
SELECT ChargeNum, 'Unknown' AS CategoryID, SUM(Hours)
FROM UnknownHours
GROUP BY ChargeNum

This UNION ALL operation will concatenate the results of the two queries into a single table, providing the desired output.

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