SQL Server能把一些常量自动转化为参数，以重用这些部分的查询计划。

SELECT FirstName, LastName, Title FROM Employees
WHERE EmployeeID = 6;
SELECT FirstName, LastName, Title FROM Employees
WHERE EmployeeID = 2;

SQL Server将会参数化上面的查询语句。

SELECT FirstName, LastName, Title FROM Employees
WHERE EmployeeID = @1

你可以运行下面的语句看看运行效果。

USE Northwind2
GO
DBCC FREEPROCCACHE;
GO
SELECT FirstName, LastName, Title FROM Employees WHERE EmployeeID = 6;
GO
SELECT FirstName, LastName, Title FROM Employees WHERE EmployeeID = 2;
GO
SELECT usecounts, cacheobjtype, objtype, [text]
FROM sys.dm_exec_cached_plans P
CROSS APPLY sys.dm_exec_sql_text (plan_handle)
WHERE cacheobjtype = 'Compiled Plan'
AND [text] NOT LIKE '%dm_exec_cached_plans%';
GO