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Select All Or One With A Single Query

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        Very often, you'll have the need to select either everything from a table or just a single record. This is the way to do both given a single query. When @EmployeeID is NULL, it will return all records from Employee, when a value is present, it will return only the value that is present.

-- Use one query to select a record given an identifier or all records when that ID is NULL

declare @EmployeeID int
set @EmployeeID = 42 -- NULL

select *
from Employee
where EmployeeID = coalesce(@EmployeeID, EmployeeID)