How to Calculate the Average of Multiple Columns in a Database Table?

Calculating Average of Multiple Table Columns

In the context of database table operations, it is often necessary to calculate average values across multiple columns. This becomes particularly useful when we need to summarize or analyze data. Let's consider a specific example to demonstrate the process.

Scenario:

We have a table named "Request" with columns representing values R1, R2, R3, R4, and R5. We want to determine the average of these values for each record in the table.

Initial Approach:

To achieve this, you initially attempted the following query:

Select Req_ID, Avg(R1+R2+R3+R4+R5) as Average
from Request
Group by Req_ID

However, this query doesn't provide the intended result. It calculates the sum of the values, not the average.

Cause:

When using the AVG aggregate function, it's important to ensure that the argument provided is an expression that evaluates to a single numeric value. In this case, you are adding the values of multiple columns, which results in a sum.

Solution:

There are two methods you can employ to calculate the average correctly:

1. Using a Subquery (2008 syntax):

SELECT *,
       (SELECT AVG(c)
        FROM   (VALUES(R1),
                      (R2),
                      (R3),
                      (R4),
                      (R5)) T (c)) AS [Average]
FROM   Request

In this approach, a subquery is used to calculate the average. The subquery selects the values from each column and treats them as separate rows. Then, the AVG aggregate function is applied to compute the average.

2. Using a UNION in the Subquery (2005 syntax):

SELECT *,
       (SELECT AVG(c)
        FROM   (SELECT R1
                UNION ALL
                SELECT R2
                UNION ALL
                SELECT R3
                UNION ALL
                SELECT R4
                UNION ALL
                SELECT R5) T (c)) AS [Average]
FROM   Request

This method uses a UNION to combine all the values from the different columns into a single column. The AVG aggregate function is then applied to the combined column to calculate the average.

Result:

Both approaches will provide the correct average values for each record in the Request table. By following any of these solutions, you can accurately calculate the average of multiple table columns and obtain the desired result.

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