The difference is COUNT(*) includes NULL values while COUNT(column_reference) doesn’t. As an example consider the two below queries for Northwind database that find the number of orders placed by each Northwind customer:
WRONG:
SELECT C.CompanyName, COUNT(*) AS NoOfOrders FROM dbo.Customers AS C LEFT OUTER JOIN dbo.Orders AS O ON C.CustomerID = O.CustomerID GROUP BY C.CompanyName ORDER BY NoOfOrders
RIGHT:
SELECT C.CompanyName, COUNT(O.OrderId) AS NoOfOrders FROM dbo.Customers AS C LEFT OUTER JOIN dbo.Orders AS O ON C.CustomerID = O.CustomerID GROUP BY C.CompanyName ORDER BY NoOfOrders
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