Functions
In many instances, you can accomplish a task
using either a stored procedure or a function. Both functions and
stored procedures can be custom defined and part of any application.
They both can be written in T-SQL or .NET and contain a header and
body. They can both accept parameters and provide output to the calling
application or user.
Creating a Function
To illustrate, let’s create a function that
accepts a pet weight and returns a table that lists the pets who exceed
that weight and their owners. The sample script is as follows:
CREATE FUNCTION GiantPets (@minWeight INTEGER)
RETURNS @retGiantPetsTable TABLE
(
pet_name VARCHAR(50) NOT NULL,
pet_weight INT NOT NULL,
owner_name VARCHAR(50) NOT NULL
)
AS
BEGIN
INSERT @retGiantPetsTable
SELECT p.pet_name, p.pet_weight,o.owner_name
FROM Pets p, Owners o
WHERE p.pet_id=o.pet_id AND p.pet_weight > @minWeight
RETURN
END
Invoking a Function
Functions are designed with the idea that they
will send their output to a query or T-SQL statement. With the
exception of scalar functions, functions are called differently than
stored procedures. Since the example function returns a table, you can
call it as part of a SELECT statement by referencing it in the FROM clause, for example:
SELECT * FROM GiantPets(50)
This will return the following result set:
pet_name pet_weight owner_name
Lady 155 Bryan
Zeus 185 Rob
System-Defined Functions
The GiantPets function is a
user-defined function. As with system stored procedures, SQL Server
comes with many built-in functions. There are so many that they are
grouped into different categories. Scalar functions use a single value or list of values, as opposed to data from multiple rows of a table. An example of a scalar function is GETDATE(), which returns the current date:
SELECT GETDATE()
Aggregate functions utilize multiple
rows of data and return a value to the user. To find the average value
for all the rows in a column, you use the AVG function as follows:
SELECT AVG(pet_weight) FROM Pets
There are also a lot of security-related functions. If you wanted to know whether the current user is a member of the sysadmin group, you would use the IS_SVRROLEMEMBER as follows:
SELECT IS_SRVROLEMEMBER('sysadmin')
This function will return 1 for true and 0 for false.
Triggers
A trigger is a special kind of
stored procedure that is executed when a special event happens within
SQL Server. An event could include the execution of a given DDL or DML
statement or a login to SQL Server. For example, if you wanted to make
sure MicroChipID was valid before allowing a new pet to be added to the Pets table, you could create a trigger on INSERT. The script follows:
USE [VetClinic]
GO
CREATE TRIGGER ValidateMicroChip
ON Pets
FOR INSERT
AS
IF EXISTS(
SELECT MicroChipID FROM MicroChips
WHERE MicroChipID IN (
SELECT MicroChipID FROM inserted)
)
RAISERROR ('The chip was found!', 16, 1)
ELSE
BEGIN
RAISERROR ('The chip was NOT found!', 16, 1)
ROLLBACK
END
GO
Now, when you attempt to insert a new pet into the table using a microchip ID that is not in the MicroChips table, an error will be issued, and the pet information will not be added to the table, as shown in the following script:
INSERT INTO Pets VALUES (8,'Sushi',5,'0034DDA')
This will result in the following result:
Msg 50000, Level 16, State 1, Procedure ValidateMicroChip, Line 13
The chip was NOT found!
Msg 3609, Level 16, State 1, Line 1
The transaction ended in the trigger. The batch has been aborted.
Since the only valid microchip ID is 34BA123, if you used this value, the result would be as follows:
Msg 50000, Level 16, State 1, Procedure ValidateMicroChip, Line 10
The chip was found!
(1 row(s) affected)
Normally, you would not have to raise an error
on a successful insert; this was done here only to help illustrate the
code path that was executed in the trigger.
If you take a look at the trigger definition, you’ll see that a table called inserted was referenced in the IF EXISTS clause. Within a DML trigger, SQL Server provides this table and a deleted table for DELETE
triggers. Together, these tables allow your triggers to do something
with the data and, depending on your business requirements, roll back
any transactions. In this sample scenario, if the chip wasn’t found,
the trigger rolled back the transaction, causing the pet information
not to be written to the table.
