1. PROCEDURES, CONDITIONAL LOGIC, EXCEPTION HANDLING, TRIGGERS
DPT621S

2. Recap on previous lessons
PHASE 1
Install SQL Server 2012 Express
Install a Sample Database
Attach a Sample Database
PHASE 2
Create an ERD
Transform the ERD into the logical and physical design
Create a database in a simple way
Create tables, alter tables, drop/re-create tables in SQL Server
PHASE 3
Create stored procedures / with input and output parameters
Modify stored procedures
Display the definitions of stored procedures
Rename stored procedures
Delete stored procedures
Recap on previous lessons

3. Recap……. Creating a Stored Procedure
USE MyDatabase
GO Begins a new batch
CREATE PROCEDURE sp_Films AS
BEGIN
SELECT filmName, filmReleaseDate, filmTime
FROM Films
Order BY filmName ASC
END
Run code…..
Find your stored procedure
Execute your stored procedure (Highlight name and exe)
Recap……. Creating a Stored Procedure

4. Recap……….. Modifying your Procedure
USE MyDatabase
GO Begins a new batch
ALTER PROCEDURE sp_Films AS
BEGIN
SELECT filmName, filmReleaseDate, filmTime
FROM Films
Order BY filmName DESC
END
Recap……….. Modifying your Procedure

5. Recap….. Deleting a Procedure
DROP Procudure sp_Films
Recap….. Deleting a Procedure

6. CONDITION LOGIC IF...ELSE Statement
In SQL Server, the IF...ELSE statement is used to execute code when a condition is TRUE, or execute different code if the condition evaluates to FALSE.
CONDITION LOGIC

7. Syntax
IF condition
{...statements to execute when condition is TRUE...}
[ ELSE
{...statements to execute when condition is FALSE...} ]

8. DECLARE @n INT = 1 IF @n = 1 PRINT ‘ONE’ ELSE IF @n = 2 PRINT ‘TWO’ ELSE PRINT ‘Wrong”
Example

9. the CASE statement has the functionality of an IF-THEN-ELSE statement
the CASE statement has the functionality of an IF-THEN-ELSE statement. You can use the CASE statement within a SQL statement.
CASE statement

10. Syntax
CASE expression
WHEN value_1 THEN result_1
WHEN value_2 THEN result_2
...
WHEN value_n THEN result_n
ELSE result
END

11. CASE WHEN condition_1 THEN result_1 WHEN condition_2 THEN result_2
CASE WHEN condition_1 THEN result_1 WHEN condition_2 THEN result_2 ... WHEN condition_n THEN result_n ELSE result END

12. Example
SELECT contact_id,
CASE website_id
WHEN 1 THEN 'TechOnTheNet.com'
WHEN 2 THEN 'CheckYourMath.com'
ELSE 'BigActivities.com'
END
FROM contacts;

13. CASE example
SELECT contact_id,
CASE
WHEN website_id = 1 THEN 'TechOnTheNet.com'
WHEN website_id = 2 THEN 'CheckYourMath.com'
ELSE 'BigActivities.com'
END
FROM contacts;

14. One thing to note is that the ELSE condition within the CASE statement is optional.
SELECT contact_id,
CASE
WHEN website_id = 1 THEN 'TechOnTheNet.com'
WHEN website_id = 2 THEN 'CheckYourMath.com'
END
With the ELSE clause omitted, if no condition was found to be true, the CASE statement would return NULL.
CASE Statement ….

15. Controls the flow of your script should an error occur
Main Focus………..
Try …..Catch Constructs
Error Functions
Use stored Procedure for error handling
Exception Handling

16. Try …..Catch Constructs Try …..Catch Construct has 2 parts
1. Try Block (Starts with: “BEGIN TRY” Ends with: “END TRY” statement)
Begin
Begin Try
End Try
End
2. Catch Block(Starts with: “BEGIN CATCH” Ends with: “END CATCH” statement)
Begin Catch
End Catch
Try …..Catch Constructs

17. Guidelines Your try block should be followed by a catch block.
Begin
Begin Try
select ‘E’
End Try
Begin Catch
End Catch
End
Try and catch block must be within a batch, stored procedure or trigger
Guidelines

18. Try….Catch constructs can be nested
Begin
Begin Try
SELECT ‘E’
SELECT ‘F’
End Try
Begin Catch
End Catch
End
You can have a nested try….catch block inside a try block or inside a catch block.

19. Error Functions which are provided by MS SQL server
They are used to capture error information.
ERROR_NUMBER()
returns the error number - type is int.
ERROR_LINE()
This returns the line number that caused error – type is int.
ERROR_SEVERITY()
returns the severity level of the error. –type is int.
ERROR_STATE()
returns the state number of the error. –type is int.
ERROR_PROCEDURE()
This returns the name of the stored procedure or trigger where the error occurred. –type is nvarchar(128)
ERROR_MESSAGE()
returns the full text of error message. The text includes the values supplied for any substitutable parameters. –type is nvarchar(4000).
Error Functions which are provided by MS SQL server

20. Example BEGIN Select 5/0 END
You will get an error …..without try….catch block
Example

21. BEGIN
Begin Try
select 5/ We put or statement inside the try…catch block
End Try
Begin Catch
select ‘Error is handled’ --so you can show any custom msg.
End Catch
END

22. We can use the build in functions to get the error details.
BEGIN
Begin Try
select 5/ We put or statement inside the try…catch block
End Try
Begin Catch
select
ERROR_LINE(),
ERROR_MESSAGE(),
ERROR_NUMBER(),
ERROR_PROCEDURE() as Proc
ERROR_SEVERITY(),
ERROR_STATE()
End Catch
END

23. Your result will show the procedure name where the error occurred.
Now we write the code in a procedure
CREATE PROCEDURE ErrorProcedure As
BEGIN
Begin Try
select 5/ We put or statement inside the try…catch block
End Try
Begin Catch
select
ERROR_LINE(),
ERROR_MESSAGE(),
ERROR_NUMBER(),
ERROR_PROCEDURE() as Proc
ERROR_SEVERITY(),
ERROR_STATE()
End Catch
END
Now execute your Procedure:
Exec ErrorProcedure
Your result will show the procedure name where the error occurred.

24. Create a common code to handle/Store Errors
Exercise

25. Answer to exercise CREATE PROCEDURE ErrorHandler As BEGIN END
Use a common stored procedure to handle errors
- Create a new procedure
- Use the stored procedure to handle errors
Step 1:
CREATE PROCEDURE ErrorHandler As
BEGIN
select
ERROR_LINE(),
ERROR_MESSAGE(),
ERROR_NUMBER(),
ERROR_PROCEDURE() as
ERROR_SEVERITY(),
ERROR_STATE()
END
Answer to exercise

26. Alter PROCEDURE ErrorProcedure As BEGIN
Step 2: Alter your procedure
Alter PROCEDURE ErrorProcedure As
BEGIN
Begin Try
select 5/ We put or statement inside the try…catch block
End Try
Begin Catch
Exec ErrorHandler --Call the error handling procedure
End Catch
END
--Run your code
Exec ErrorProcedure --Execute your procedure

27. TRIGGERS Is a special kind of procedure……
DML triggers (Table events: insert, update and delete associated with tables or views)
Can be AFTER or INSTEAD OF
You should know which table or view you want to attach the trigger to.
TRIGGERS

28. -- SQL Server Syntax Trigger on an INSERT, UPDATE, or DELETE statement to a table or view (DML Trigger) CREATE TRIGGER [ schema_name . ]trigger_name ON { table | view } [ WITH <dml_trigger_option> [ ,...n ] ] { FOR | AFTER | INSTEAD OF } { [ INSERT ] [ , ] [ UPDATE ] [ , ] [ DELETE ] } [ WITH APPEND ] [ NOT FOR REPLICATION ] AS { sql_statement [ ; ] [ ,...n ] | EXTERNAL NAME <method specifier [ ; ] > } <dml_trigger_option> ::= [ ENCRYPTION ] [ EXECUTE AS Clause ] <method_specifier> ::= assembly_name.class_name.method_name
Trigger Syntax

29. Example: CREATING A ‘AFTER’ Trigger
Use AdvantureWorks -- your database Go
Create Trigger MyContacts
ON Contacts --Which table or view the trigger is created for
AFTER insert, update, delete
-- After which event? You can have 1 or more events AS
BEGIN
--What do we want our trigger to do?
-- We can print a msg to say something happened to our Contacts table
Print ‘something happened to our Contacts table’
END GO
Run your code………………………
Completed successfully

30. We need to trigger our insert or update or delete events in our table Contacts.
INSERT Into Contacts(list of columns,….)
Values(list of values, …)
--Update
UPDATE Contacts
SET DOD = GETDATE() Date of death
WHERE ID = 999
--Delete
DELETE FROM CONTACTS
Execute your code….
3 messages will be displayed
Test Trigger

31. Modify your trigger Use AdvantureWorks -- your database Go
ALTER Trigger MyContacts
ON Contacts --Which table or view the trigger is created for
AFTER insert, undate, delete
-- After which event? You can have 1 or more events AS
BEGIN
--What do we want our trigger to do?
-- We can print a msg to say something happened to our Contacts table
Print ‘Data was changed in the Contacts table’
END GO
Modify your trigger

32. Use AdvantureWorks -- your database Go Drop Trigger MyContacts
Delete your Trigger

33. EXAMPLE: Create a INSTEAD OF Trigger
Use AdvantureWorks -- your database Go
Create Trigger MyContactsTwo
ON Contacts --Which table or view the trigger is created for
INSTEAD OF insert
-- After which event? You can have 1 or more events AS
BEGIN
--Custom error message with msg, severity and state
RAISERROR (‘No more contacts can be inserted, 16, 1)
END GO
Run your code………………………
Completed successfully
Now test the code
EXAMPLE: Create a INSTEAD OF Trigger

34. USE AdventureWorks GO
INSERT INTO Contacts(list of columns,….)
Values(list of values, …)
SELECT * FROM Contacts
WHERE ID = 999
--Will not return any records
Error msg will be displayed

35. Exercise Create a trigger to validate data in your database.
You have created a date of death for this contact so you want to make sure any update is valid.
The criteria is: our trigger should check that the ID you provide for the person’s DOD is not null then the trigger should stop the event.
Exercise

36. Answer Use AdvantureWorks -- your database Go
Step 1: Create a new trigger
Use AdvantureWorks -- your database Go
Create Trigger CastContact
ON Contacts --Which table or view the trigger is created for
AFTER insert
-- After which event? You can have 1 or more events AS
BEGIN
--Custom error message with msg, severity and state
IF EXIST(
SELECT *
FROM Contacts AS a
INNER JOIN insertedTabl AS I
ON a.ID = i.CastID
WHERE DOD IS NOT NULL)
BEGIBN
RAISERROR(‘ Sorry but that person has expired’, 16, 1)
ROLLBACK TRANSACTION -- This will cancel the insert if it happened.
RETURN If I don’t want the procedure to end
END GO
Execute script……
Test your trigger by trying to insert some data…..using the deceased person