Execute Dynamic SQL commands in SQL Server

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In some applications having hard-coded SQL statements is not appealing, because of the dynamic nature of the queries being issued against the database server. Because of this sometimes there is a need to dynamically create a SQL statement on the fly and then run that command. This can be done quite simply from the application perspective where the statement is built on the fly whether you are using ASP.NET , ColdFusion or any other programming language. But how do you do this from within a SQL Server stored procedure? SQL Server offers a few ways of running a dynamically built SQL statement. These ways are: Writing a query with parameters Using EXEC Using sp_executesql Writing a query with parameters This first approach is pretty straightforward if you only need to pass parameters into the WHERE clause of your SQL statement. Let’s say we need to find all records from the Customers table where City = ‘London’. This can be done easily as the following example shows.
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Every SQL developer must know about sql case command

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suppose We need a stored procedure that can be called by an application but the user wants to be able to sort by either first name or last name. One would be tempted to use dynamic SQL to solve this problem, but we can use CASE to create a dynamic SQL equivalent

CREATE PROCEDURE dbo.getCustomerData 
@sortby VARCHAR(9), 
@sortdirection CHAR(4) AS SET nocount ON SELECT customerid, firstname, lastname, statecode, statedescription, totalsales 
FROM dbo.Customer ORDER BY 
	CASE @sortdirection WHEN 'asc' 
		THEN CASE @sortby WHEN 'firstname' 
		THEN firstname WHEN 'lastname' 
		THEN lastname 
	END 
END ASC, 
	CASE @sortdirection WHEN 'desc' 
	THEN CASE @sortby WHEN 'firstname' 
	THEN firstname WHEN 'lastname' 
	THEN lastname 
END END 
DESC 
GO

How to execute

EXEC dbo.getCustomerData 'lastname', 'desc'

A final requirement has crossed our desk. We need to modify the stored procedure to search customers by a specific state. If the state is omitted, we should return customers for all states.

ALTER PROCEDURE dbo.getCustomerData 
@sortby VARCHAR(9), 
@sortdirection CHAR(4),
 @statecode VARCHAR(2) = NULL AS SET nocount ON SELECT customerid, firstname, lastname, statecode, statedescription, totalsales 
 FROM dbo.Customer 
 WHERE statecode = CASE WHEN @statecode IS NOT NULL 
 THEN @statecode 
 ELSE statecode 
 END ORDER BY 
 CASE @sortdirection WHEN 'asc' 
 THEN CASE @sortby WHEN 'firstname' 
 THEN firstname WHEN 'lastname' THEN lastname END END 
 ASC, 
 CASE @sortdirection WHEN 'desc' 
 THEN CASE @sortby WHEN 'firstname' 
 THEN firstname WHEN 'lastname' 
 THEN lastname 
 END 
 END 
 DESC GO 

EXEC dbo.getCustomerData 'lastname', 'desc', 'MA'

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