How to Count Distinct Program Names in SQL Server Using COUNT(DISTINCT)?

Efficiently Counting Unique Program Names in SQL Server

Determining the number of unique entries within a dataset is a common task in data analysis. SQL Server offers a straightforward method to count distinct values using the COUNT(DISTINCT) function. This is particularly useful when dealing with large datasets where understanding unique occurrences is crucial.

Let's consider the cm_production table, which stores information about code deployments including ticket numbers, program types, program names, and push numbers. Our objective is to determine the count of unique program names, categorized by program type and push number.

An initial, incorrect approach might look like this:

DECLARE @push_number INT;
SET @push_number = [HERE_ADD_NUMBER];

SELECT DISTINCT COUNT(*) AS Count, program_type AS [Type] 
FROM cm_production 
WHERE push_number=@push_number 
GROUP BY program_type

This query, while seemingly correct, actually counts all program names, not just the unique ones. The key to obtaining the correct count lies in utilizing the COUNT(DISTINCT <expression>)</expression> function. This function specifically returns the number of unique, non-null values for a given expression.

The corrected SQL query is as follows:

SELECT program_type AS [Type],
       COUNT(DISTINCT program_name) AS [Count]
FROM   cm_production
WHERE  push_number = @push_number
GROUP  BY program_type

This refined query leverages COUNT(DISTINCT program_name) to accurately count the distinct program names for each program type and the specified push_number. This provides a precise representation of the unique code deployments within the database.

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