Can any one tell me the difference between select count(*) and count(1) in sql server?

 In SQL Server, both COUNT(*) and COUNT(1) are used to count the number of rows in a table or query result. However, there is a subtle difference between them. Let’s break it down:

1. COUNT(*)

• Definition: COUNT(*) counts all rows in a result set, including rows with NULL values in any of the columns.
• Performance: SQL Server doesn't need to check the content of the columns when using COUNT(*). It simply counts rows, regardless of their content.
• Common Use Case: Typically used when you need to count all rows, regardless of their column values (including NULL values).

Example:

SELECT COUNT(*) FROM Employees;

This will return the total number of rows in the Employees table, including those rows where all or some of the columns may have NULL values.

2. COUNT(1)

• Definition: COUNT(1) counts the number of rows, but only counts rows where the expression (in this case, 1) is non-NULL. Since 1 is a constant that is never NULL, COUNT(1) effectively counts all rows.
• Performance: While it may seem like COUNT(1) does more work (checking the expression 1 for each row), it performs essentially the same as COUNT(*) in SQL Server. SQL Server is optimized to handle both and can treat them in a very similar way under the hood.
• Common Use Case: It's sometimes used to make a query look more specific, but it does not change the result from COUNT(*).

Example:

SELECT COUNT(1) FROM Employees;

This will also return the total number of rows in the Employees table, as 1 is never NULL.

Key Differences:

• Logical Difference: There is no significant logical difference between COUNT(*) and COUNT(1) in SQL Server. Both count all rows, regardless of their column values, and both will include rows that have NULL values in any of their columns.

• Performance: Historically, there might have been slight performance differences in some database systems, but in SQL Server, both COUNT(*) and COUNT(1) are optimized to be effectively equivalent. The engine treats them the same way and performs the same operations behind the scenes.

Conclusion:

In SQL Server, COUNT(*) and COUNT(1) can be used interchangeably with no meaningful difference in functionality or performance. The use of COUNT(*) is more common, as it clearly expresses the intention to count all rows in a table.