Tài liệu Creating User-Defined Functions pdf

• Inline table-valued functions Inline table-valued functions return an object of the table type.. • Multistatement table-valued functions Multistatement table-valued functions retur

Creating User-Defined Functions

You can create your own user-defined functions in SQL Server For example, you might want to create your own function to compute the discounted price given the original price and factor to multiply that price by You create a function using the CREATE

FUNCTION statement There are three types of userdefined functions:

• Scalar functions Scalar functions return a single value The returned value can be

of any data type except text, ntext, image, cursor, table, timestamp, and user-defined data types

• Inline table-valued functions Inline table-valued functions return an object of the table type You can think of a table as a regular database table, except it is stored

in memory An inline table-valued function can return the results retrieved by only

a single SELECT statement

• Multistatement table-valued functions Multistatement table-valued functions return an object of the table type Unlike an inline table-valued function, a

multistatement table-valued function can contain multiple T-SQL statements You'll see examples of these three types of functions in the following sections

Using Scalar Functions

Scalar functions return a single value Listing 4.2 shows the DiscountPrice.sql script that creates the DiscountPrice() function, which returns the original price of an item

multiplied by a discount factor These values are passed as parameters to the

DiscountPrice() function You can load this file into Query Analyzer and run it

Listing 4.2: DISCOUNTPRICE.SQL

/*

DiscountPrice.sql creates a scalar function to

return the new price of an item given the original

price and a discount factor

*/

CREATE FUNCTION DiscountPrice(@OriginalPrice money, @Discount float)

RETURNS money

AS

BEGIN

RETURN @OriginalPrice * @Discount

END

The parameters to the function are placed in brackets after the name of the function in the CREATE FUNCTION statement

Warning Make sure you select the Northwind database from the drop-down list box on

the Query Analyzer toolbar before running the script That way, the function is created in the Northwind database

You can also create functions using Enterprise Manager You do this by clicking the right mouse button on the User Defined Functions node in the Databases folder and selecting New User Defined Function You can then cut and paste the contents of

DiscountPrice.sql into the Enterprise Manager properties dialog box, as shown in Figure 4.4

Figure 4.4: Using Enterprise Manager to define a function

You can view and modify a function by double-clicking the function name in Enterprise Manager You can also delete a function using Enterprise Manager The Object Browser

of Query Analyzer allows you to view, modify, and delete functions as well

Tip You can also delete a function using the DROP FUNCTION statement, and you can modify a function using the ALTER FUNCTION statement

Once you've created the function, you can call it When calling a scalar function, you use the following syntax:

owner.functionName

Where owner is the database user who owns the function, and functionName is the name

of the function

Let's say you created the DiscountPrice() function using the dbo user, then you call that function using dbo.DiscountPrice() The following example returns 3.0000, which is 10 * 0.3:

SELECT dbo.DiscountPrice(10, 0.3);

As with any other function, you can pass a column to DiscountPrice() The following example returns 5.4000 and 18.0000; 5.4000 is 18.0000 * 0.3:

SELECT dbo.DiscountPrice(UnitPrice, 0.3), UnitPrice

FROM Products

WHERE ProductID = 1;

You can of course also pass variables as parameters to a function As before, this

example returns 5.4000 and 18.0000:

DECLARE @MyDiscountFactor float

SET @MyDiscountFactor = 0.3

SELECT dbo.DiscountPrice(UnitPrice, @MyDiscountFactor), UnitPrice

FROM Products

WHERE ProductID = 1;

Using Inline Table-Valued Functions

An inline table-valued function returns an object of the table type, which is populated using a single SELECT statement Unlike a scalar function, an inline table-valued

function doesn't contain a body of statements placed within BEGIN and END statements Instead, only a single SELECT statement is placed within the function

For example, Listing 4.3 shows the ProductsToBeReordered.sql script that creates the ProductsToBeReordered() function This function returns a table containing the rows from the Products table with a UnitsInStock column value less than or equal to the

reorder level parameter passed to the function

Listing 4.3: PRODUCTSTOBEREORDERED.SQL

/*

ProductsToBeReordered.sql creates an inline table-valued function to

return the rows from the Products table whose UnitsInStock column

is less than or equal to the reorder level passed as a parameter

to the function

*/

CREATE FUNCTION ProductsToBeReordered(@ReorderLevel int)

RETURNS table

AS

RETURN

(

SELECT *

FROM Products

WHERE UnitsInStock <= @ReorderLevel

)

Unlike a scalar function, you don't have to add the owner when calling an inline table-valued function You use a SELECT statement to read the table returned by the function

as you would any other table For example, the following SELECT statement displays all the rows and columns returned by the function call ProductsToBeReordered(10):

SELECT *

FROM ProductsToBeReordered(10);

You can of course also display only selected columns and rows from the table returned

by an inline table-valued function For example:

SELECT ProductID, ProductName, UnitsInStock

FROM ProductsToBeReordered(10)

WHERE ProductID <= 50;

Figure 4.5 shows the results of this SELECT statement

Figure 4.5: Using an inline table-valued function

Using Multistatement Table-Valued Functions

Multistatement table-valued functions return an object of the table type Unlike an inline table-valued function, a multistatement table-valued function can contain multiple T-SQL statements, and allow you to build complex functions

For example, Listing 4.4 shows the ProductsToBeReordered2.sql script that creates the ProductsToBeReordered2() function This function returns a table containing the

ProductID, ProductName, and UnitsInStock columns from the Products table with a UnitsInStock column value less than or equal to the reorder level parameter In addition,

a new column named Reorder is added to the table, which contains the word Yes or No, depending on whether the product must be reordered

Listing 4.4: PRODUCTSTOBEREORDERED2.SQL

/*

ProductsToBeReordered2.sql creates an inline table-valued

function that returns the rows from the Products table

whose UnitsInStock column is less than or equal to the

reorder level passed as a parameter to the function

*/

CREATE FUNCTION ProductsToBeReordered2(@ReorderLevel int)

RETURNS @MyProducts table

(

ProductID int,

ProductName nvarchar(40),

UnitsInStock smallint,

Reorder nvarchar(3)

)

AS

BEGIN

retrieve rows from the Products table and

insert them into the MyProducts table,

setting the Reorder column to 'No'

INSERT INTO @MyProducts

SELECT ProductID, ProductName, UnitsInStock, 'No'

FROM Products;

update the MyProducts table, setting the

Reorder column to 'Yes' when the UnitsInStock

column is less than or equal to @ReorderLevel

UPDATE @MyProducts

SET Reorder = 'Yes'

WHERE UnitsInStock <= @ReorderLevel

RETURN

END

As with an inline table-valued function, you don't have to add the owner when calling an inline table-valued function You use a SELECT statement to read the table returned by the function as you would any other regular database table For example, the following SELECT statement displays all the rows and columns returned by the function call ProductsToBeReordered2(20):

SELECT *

FROM ProductsToBeReordered2(20);

Figure 4.6 shows the results of this SELECT statement

Figure 4.6: Using a multistatement table-valued function

In the next section, you'll learn how to use stored procedures