A stored procedure is a group of Transact-SQL statements compiled into a single execution plan. stored procedures return data in four ways:
• Output parameters, which can return either data (such as an integer or character value) or a cursor variable (cursors are result sets that can be retrieved one row at a time).
• Return codes, which are always an integer value.
• A result set for each SELECT statement contained in the stored procedure or any other stored procedures called by the stored procedure.
• A global cursor that can be referenced outside the stored procedure. Stored procedures assist in achieving a consistent implementation of logic across applications. The SQL statements and logic needed to perform a commonly performed task can be designed, coded, and tested once in a stored procedure. Each application needing to perform that task can then simply execute the stored procedure. Coding business logic into a single stored procedure also offers a single point of control for ensuring that business rules are correctly enforced. Stored procedures can also improve performance. Many tasks are implemented as a series of SQL statements. Conditional logic applied to the results of the first SQL statements determines which subsequent SQL statements are executed. If these SQL statements and conditional logic are written into a stored procedure, they become part of a single execution plan on the server. The results do not have to be returned to the client to have the conditional logic applied; all of the work is done on the server. The IF statement in this example shows embedding conditional logic in a procedure to keep from sending a result set to the application: IF (@QuantityOrdered < (SELECT QuantityOnHand
WHERE PartID = @PartOrdered) )
-- SQL statements to update tables and process order.
-- SELECT statement to retrieve the IDs of alternate items -- to suggest as replacements to the customer.
Applications do not need to transmit all of the SQL statements in the procedure: they have to transmit only an EXECUTE or CALL statement containing the name of the procedure and the values of the parameters. Stored procedures can also shield users from needing to know the details of the tables in the database. If a set of stored procedures supports all of the business functions users need to perform, users never need to access the tables directly; they can just execute the stored procedures that model the business processes with which they are familiar. An illustration of this use of stored procedures is the SQL Server system stored procedures used to insulate users from the system tables. SQL Server includes a set of system stored procedures whose names usually start with sp_. These system stored procedures support all of the administrative tasks required to run a SQL Server system. You can administer a SQL Server system using the Transact-SQL administration-related statements (such as CREATE TABLE) or the system stored procedures, and never need to directly update the system tables.
DROP PROCEDURE OrderSummary
CREATE PROCEDURE OrderSummary @MaxQuantity INT OUTPUT AS
-- SELECT to return a result set summarizing
-- employee sales.
SELECT Ord.EmployeeID, SummSales = SUM(OrDet.UnitPrice * OrDet.Quantity) FROM Orders AS Ord
JOIN [Order Details] AS OrDet ON (Ord.OrderID = OrDet.OrderID) GROUP BY Ord.EmployeeID
ORDER BY Ord.EmployeeID
-- SELECT to fill the output parameter with the
-- maximum quantity from Order Details.
SELECT @MaxQuantity = MAX(Quantity) FROM [Order Details]
-- Return the number of all items ordered.
RETURN (SELECT SUM(Quantity) FROM [Order Details])
-- Test the stored procedure.
-- DECLARE variables to hold the return code
-- and output parameter.
DECLARE @OrderSum INT
DECLARE @LargestOrder INT
-- Execute the procedure, which...
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