Returning Maximum Values for Specific Columns in a Single Query

In this article, we will explore how to return only the maximum values for specific columns from a database table. This is often referred to as “aggregating” or “grouping” data.

Understanding the Problem

Suppose we have a database table called tblDemoOrdinalNumbers that contains columns such as Kitchen, Bar, Pizzeria, and Barbecue. We want to retrieve the maximum value for each of these columns. One approach is to write separate queries for each column, but this can become cumbersome and inefficient.

Using a Separate Column for Each Max Value

One way to solve this problem is to create a separate column in our query that contains the maximum value for each column. We can use the MAX function with a conditional statement (CASE) to achieve this.

SELECT MAX(CASE WHEN Kitchen = 1 THEN LastOrdinalNumber END) AS Kitchen,
       MAX(CASE WHEN Bar = 1 THEN LastOrdinalNumber END) AS Bar,
       MAX(CASE WHEN Pizzeria = 1 THEN LastOrdinalNumber END) AS Pizzeria,
       MAX(CASE WHEN Barbecue = 1 THEN LastOrdinalNumber END) AS Barbecue
FROM tblDemoOrdinalNumbers;

This query will return four columns, each containing the maximum value for a specific column. The CASE statement checks the condition for each column and returns the corresponding value if true.

Joining Back to the Original Table

If we want to retrieve the data in rows instead of just the maximum values, we can join back to the original table using an inner join.

SELECT d.*
FROM (SELECT MAX(CASE WHEN Kitchen = 1 THEN LastOrdinalNumber END) AS Kitchen,
        MAX(CASE WHEN Bar = 1 THEN LastOrdinalNumber END) AS Bar,
        MAX(CASE WHEN Pizzeria = 1 THEN LastOrdinalNumber END) AS Pizzeria,
        MAX(CASE WHEN Barbecue = 1 THEN LastOrdinalNumber END) AS Barbecue
     FROM tblDemoOrdinalNumbers) x
JOIN tblDemoOrdinalNumbers d ON d.LastOrdinalNumber IN (x.Kitchen, x.Bar, x.Pizzeria, x.Barbecue)

In this query, we use a subquery to compute the maximum values for each column. We then join back to the original table tblDemoOrdinalNumbers using an inner join on the condition that the LastOrdinalNumber value is in one of the four columns.

How it Works

The key insight behind this approach is to use a separate column to store the maximum value for each column, and then join back to the original table to retrieve the actual data. This allows us to avoid writing multiple separate queries for each column.

Tips and Variations

Here are some additional tips and variations:

• To make the query more efficient, we can add an INDEX on the columns used in the CASE statement.
• If there are many columns that we want to retrieve the maximum value for, we can use a table-valued function (TVF) or a Common Table Expression (CTE) to compute the values once and then join back to the original table.
• To make the query more readable, we can break up the CASE statement into multiple lines using line breaks.

Example Use Case

Suppose we have a database table called tblDemoOrdinalNumbers that contains the following data:

Using the query above, we can retrieve the following maximum values:

This shows us that the maximum value for Kitchen is 10, for Bar is 20, for Pizzeria is 30, and for Barbecue is 40.

Last modified on 2024-07-13