Understanding SUM Function in SQL and Removing Duplicates

As a technical blogger, I’m often asked about various aspects of SQL queries, including the SUM function. In this article, we’ll explore how to use the SUM function in SQL to calculate values from one column based on another column having the same value.

What is SUM Function in SQL?

The SUM function in SQL is used to calculate the sum of a set of values within a database table. It takes a column name as an argument and returns the total value of all records in that column.

Example:

SELECT SUM(column2) FROM Table;

This query will return the sum of all values in the column2 column.

Using SUM Function with GROUP BY

When you want to calculate sums for groups of data based on one column, you can use the GROUP BY clause along with the SUM function. The basic syntax is:

SELECT column1, SUM(column2) FROM Table GROUP BY column1;

This query will return the sum of all values in the column2 column for each group identified by the column1.

Removing Duplicate Rows

However, when using the GROUP BY clause with the SUM function, you might notice that duplicate rows are included in the output. This is because SQL treats identical data as equal.

To remove these duplicates, you can add an additional GROUP BY clause with a column that uniquely identifies each row.

Example:

SELECT column1, SUM(column2) FROM Table GROUP BY column1, column2;

However, this approach might not work well when the unique identifier is not explicitly mentioned in your SQL query. A better approach would be to use subqueries or join with another table that contains the duplicate rows.

How to Remove Duplicate Rows

One way to remove duplicates from a SQL query is by using the DISTINCT keyword along with the GROUP BY clause.

Example:

SELECT DISTINCT column1, SUM(column2) FROM Table GROUP BY column1;

This approach will return only unique rows that have the same values in both columns.

Practical Example

Let’s use a practical example to illustrate this concept. Suppose we have a table exam_results with two columns: subject and score.

We want to calculate the sum of scores for each subject.

SELECT subject, SUM(score) FROM exam_results GROUP BY subject;

This query will return:

As we can see, duplicate rows are removed from the output.

Conclusion

In this article, we explored how to use the SUM function in SQL along with the GROUP BY clause to calculate sums for groups of data. We also discussed ways to remove duplicates from the output and provided practical examples to illustrate these concepts. By mastering the use of SQL functions like SUM, you can efficiently analyze and summarize large datasets.

Additional Tips

• Always specify the column name in the GROUP BY clause.
• Use the DISTINCT keyword along with GROUP BY to remove duplicate rows.
• Consider using subqueries or joins if you need more complex logic.

Last modified on 2024-01-31