sql distinct count

In SQL, the `COUNT` function is used to count the number of rows in a table or the number of non-NULL values in a specific column. However, sometimes you need to count only unique values. This is where the `DISTINCT` keyword comes into play. Using `DISTINCT` with `COUNT` ensures that each unique value is counted only once, providing a more accurate representation of the variety of data in a column. For example, if you have a list of customer IDs, using `COUNT(DISTINCT customer_id)` will give you the total number of unique customers, not the total number of rows with customer IDs.Imagine you have a sales table with multiple entries for the same product. If you simply use `COUNT(*)`, you'll get the total number of sales records. But if you want to know how many different products were sold, you need to use `COUNT(DISTINCT product_name)`. This gives you a count of unique products, not the total number of sales for each product.The `DISTINCT` keyword filters out duplicate rows before the `COUNT` function operates. This is a powerful tool for data analysis, allowing you to understand the variety of data within a column without being misled by repeated entries. It's essential for tasks like calculating the number of unique customers, products, or any other distinct category within your data.Using `DISTINCT` with `COUNT` is a standard SQL practice. It's a fundamental technique for obtaining accurate counts of unique values, which is crucial for various reporting and analysis tasks.