Problem>> How to write Dynamic sql Query?
Solution>> Go to this link http://www.sqlusa.com/bestpractices/dynamicsql/
The best place to learn About dynmaic sql
 or see below

USE AdventureWorks2008;

EXEC ('SELECT * FROM Production.Product')    -- tsql constant string execution ------------

DECLARE @TableName sysname ='Sales.SalesOrderHeader'

EXECUTE ('SELECT * FROM '+@TableName)        -- T-SQL string with variable - dynamic ------------

EXECUTE sp_executeSQL N'SELECT * FROM Purchasing.PurchaseOrderHeader'
------------
DECLARE @SQL varchar(256); SET @SQL='SELECT * FROM Production.Product'

EXEC (@SQL)                -- SQL Server 2008 dynamic SQL query execution
------------

DECLARE @SQL varchar(256), @Table sysname;

SET @SQL='SELECT * FROM'; SET @Table = 'Production.Product'

SET @SQL = @SQL+' '+@Table

PRINT @SQL     -- for debugging dynamic SQL prior to execution of generated static code

EXEC (@SQL)    -- Microsoft dynamic SQL execution - SQL Server 2005 execute dynamic SQL

-------------- Dynamic queries with local variables and parameter(s) - t-sql parameterized query

-- Parameters are NOT ALLOWED everywhere in an SQL statement - hence dynamic SQL

DECLARE @Color varchar(16) = 'Yellow'

SELECT Color=@Color, ProductCount=COUNT(Color)

FROM AdventureWorks2008.Production.Product

WHERE Color = @Color

/*    Color    ProductCount

      Yellow      36              */------------

-- SQL Server 2012 sp_executeSQL usage with input and output parameters (2008/2005)

DECLARE @SQL NVARCHAR(max), @ParmDefinition NVARCHAR(1024)

DECLARE @ListPrice money = 2000.0, @LastProduct varchar(64)

SET @SQL =       N'SELECT @pLastProduct = max(Name)

                   FROM AdventureWorks2008.Production.Product

                   WHERE ListPrice >= @pListPrice'

SET @ParmDefinition = N'@pListPrice money,

                        @pLastProduct varchar(64) OUTPUT'

EXECUTE sp_executeSQL      -- Dynamic T-SQL

            @SQL,

            @ParmDefinition,

            @pListPrice = @ListPrice,

            @pLastProduct=@LastProduct OUTPUT

SELECT [ListPrice >=]=@ListPrice, LastProduct=@LastProduct

/* ListPrice >=   LastProduct

2000.00     Touring-1000 Yellow, 60 */
------------

-- 3-part naming execution of dynamic SQL - Specifying database (USE dbname)

EXEC AdventureWorks2008R2.dbo.sp_executeSQL N'exec sp_help'

------------

For more details about other operations please visit the below link

Courtsy :http://www.sqlusa.com/bestpractices/dynamicsql/