How to Retrieve '0' from MySQL's SUM Function When No Values Exist
The SUM function in MySQL offers a convenient way to aggregate numerical values. However, when no matching rows are found during the query, the SUM function typically returns a NULL value. For some use cases, it may be more desirable to return '0' instead of NULL.
Utilizing COALESCE to Resolve the Issue
The solution to this problem lies in the COALESCE function. COALESCE allows you to specify a default value to be returned when the first argument evaluates to NULL.
Consider the following example:
SELECT COALESCE(SUM(Column_1),0) FROM Table WHERE Column_2 = 'Test'
In this query, COALESCE wraps around the SUM function. If the SUM of Column_1 for rows where Column_2 equals 'Test' returns a NULL value, COALESCE will instead provide '0'.
Demonstration and More Information
Visit http://www.sqlfiddle.com/#!2/d1542/3/0 for a live demonstration of the solution.
To delve deeper into the nuances of COALESCE, consider the following scenario involving three tables with different value distributions:
By applying the following query:
SELECT 'foo' as table_name, 'mixed null/non-null' as description, 21 as expected_sum, COALESCE(SUM(val), 0) as actual_sum FROM foo UNION ALL SELECT 'bar' as table_name, 'all non-null' as description, 21 as expected_sum, COALESCE(SUM(val), 0) as actual_sum FROM bar UNION ALL SELECT 'baz' as table_name, 'all null' as description, 0 as expected_sum, COALESCE(SUM(val), 0) as actual_sum FROM baz
You will obtain the following results:
TABLE_NAME | DESCRIPTION | EXPECTED_SUM | ACTUAL_SUM |
---|---|---|---|
foo | mixed null/non-null | 21 | 21 |
bar | all non-null | 21 | 21 |
baz | all null | 0 | 0 |
As you can observe, the COALESCE function efficiently returns '0' for the 'baz' table, where all values are NULL.
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