1. Stored Procedures, Transactions, and Error-Handling
4/23/2017
Stored Procedures, Transactions, and Error-Handling
BIT275/276
Instructor: Craig Duckett

2. Stored Procedure vs. View
A Stored Procedure:
accepts parameters
can NOT be used as building block in a larger query
can contain several statements, loops, IF ELSE, etc.
can perform modifications to one or several tables
can NOT be used as the target of an INSERT, UPDATE or DELETE statement.
A View:
does NOT accept parameters
CAN be used as building block in a larger query
can contain only one single SELECT query
can NOT perform modifications to any table
but can (sometimes) be used as the target of an INSERT, UPDATE or DELETE statement.

3. Stored Procedure vs. Function
Basic Difference
Function must return a value but in Stored Procedure it is optional ( Procedure can return zero or n values).
Functions can have only input parameters for it whereas Procedures can have input/output parameters .
Functions can be called from Procedure whereas Procedures cannot be called from Function.
Advance Difference
Procedure allows SELECT as well as DML(INSERT/UPDATE/DELETE) statement in it whereas Function allows only SELECT statement in it.
Procedures can not be utilized in a SELECT statement whereas Function can be embedded in a SELECT statement.
Stored Procedures can NOT be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section whereas Function can be.
The most important feature of stored procedures over function is to retention and reuse the execution plan while in case of function it will be compiled every time.
Functions that return tables can be treated as another rowset. This can be used in JOINs with other tables.
Inline Function can be though of as views that take parameters and can be used in JOINs and other Rowset operations.
Exception can be handled by TRY-CATCH block in a Procedure whereas a TRY-CATCH block can NOT be used in a Function.
We can go for Transaction Management in Procedure whereas we can't go in Function.

4. Scripts v. Stored Procedures
Script: Text file of SQL commands
Stored Procedure: SQL commands stored in database itself
SPROC’s have more capabilities than a script

5. BATCH Batch is a logical group of SQL statements
Run-time error will halt execution only of FURTHER steps
Can break up multiple steps using GO
Not available in all tools
GO causes editing tool to send statements to that point for execution
GO isn’t sent to SQL Server

6. Format of SPROC’s CREATE PROCEDURE <name> <parameter list>AS
<instructions to execute>

7. EXECUTE EXEC(cute) statement EXEC(cute) stored procedure name
Statement or sproc runs in it’s own scope
Can’t ‘share’ variables directly
User’s security rules apply
Can’t be used in User Defined Function (UDF)

8. Uses of Stored Procedures
For returning data (select)
For editing data
For calculations

9. Parameters Method for sending data into and from a stored procedure
INPUT parameters are values sent in
OUTPUT parameters are values returned
Must have a holding space (variable) for the returned data
Defined before start of procedure (AS)

10. Declaring Parameters Include name and datatype
Default value is optional
Without a default value, parameter is required
Direction is optional (input is default)
An output parameter must have direction specified

11. Sample Input Parameter
CREATE PROC upFindStudent
@SID char(9) AS
SELECT *
FROM Persons
Where

12. Sample Output Parameter
CREATE PROC char(9) OUTPUT AS FROM Students_T

13. Return Values Result of stored procedure indicates success or failure
Non-zero value indicates a problem
Must be an integer
Different from an output parameter
Output parameter is about data
RETURN <value>
Causes immediate exit

14. Variables Create using DECLARE Need to start with ‘@’
Can use SQL data types or custom data types
varchar(50)

15. Variable Assignment SET is usually used similar to procedural language
SELECT is usually used when getting a value from a query
FROM Assignments

16. Decision Making SQL supports two structures for branching:IF
CASE
Both structures are similar to other languages (IF … THEN, SELECT CASE)
Both structures tend to have specific places where used

17. IF Blocks IF … ELSE No end if
Stored Procedures
4/23/2017
IF Blocks
IF … ELSE
No end if
Need to use Begin/End if have more than one instruction to execute
IF StartDate < EndDate
Begin …
End
ELSE

18. Simple Case Statement CASE Similar to SELECT CASE
Compares one value to different cases
CASE Category
WHEN ‘pop_comp’ THEN ‘Popular Computing’
WHEN ‘mod_cook’ THEN ‘Modern Cooking’
END

19. Searched CASE No test expression Each WHEN has a boolean test CASE END
WHEN Points >= 90 THEN ‘A’
WHEN Points < 90 AND Extra > 0
THEN ‘A’
END

20. Looping (While) Typically used with a CURSOR
Cursor data type allows a table to be stored in memory and each row/field to be accessed
BREAK allows early exit from loop
CONTINUE forces control to start of loop
Working with sets is preferred over loops (SQL is about sets)

21. Finding Identity Values
When need to find the value used to identify the last row added
Scope_Identity
Ident_Current

22. @@Identity System variable, created/maintained automatically
Returns the last identity value used as a result of INSERT or SELECT INTO
Not limited to current scope; may not get correct value
Returns Null if operation failed or a value wasn’t generated
Returns last number created if multiple inserts occur (i.e. SELECT INTO)

23. Scope_Identity()
Return the last identity values generated in any table in the current session.
Returns values inserted only within the current scope
Not affected by other operations.

24. Ident_Current() Not limited by scope and session;
Limited to a specified table (table name specified as an argument value).

25. @@Rowcount System variable, created/maintained automatically
Number of rows returned or affected by the last statement
0 (zero) is often used as a logical test
If no records found for where clause, notify system or process

26. Errors Errors can occur because of SQL statement
Invalid syntax, or data type
Errors can also reflect business rules
Data doesn’t match requirements

27. @@Error System variable, created/maintained automatically
Stored Procedures
4/23/2017
System variable, created/maintained automatically
Value set after each SQL statement;
0 (zero) means statement was successful
Number other than zero is typically a specific error
Can store value in variable and test

28. Try/Catch Similar to .Net languages Need to include BEGIN/END
BEGIN TRY
<code>
END TRY
BEGIN CATCH
<error handling code>
END CATCH

29. Raise Error Used to send information to calling program Syntax:
RaisError (Message string OR Message ID, Severity, State)
Severity – <14 information; warning or user can correct; 20+ fatal
State – way to differentiate problems if needed; typically use 1
RAISERROR (50001,16,1)

30. Error Message Message ID or String
Use ID if have custom or TSQL error to use
Use String for ‘on the fly’ message
Stored Error Messages are server-specific
Can add message to server
ID Number must be greater than 50000

31. Custom Error Messages
Messages can include a parameter with % to allow addition to message
‘D’ – signed integer
‘O’ – unsigned octal
‘P’ – pointer
‘S’ – string
‘U’ – unsigned integer
‘X’ or ‘x’ – unsigned hexadecimal

32. Severity & State 1 – 18: Informational (range can vary – not exact)
11 – 16 typically raise error at client
19 – 25: Severe error
20+ is fatal error and connection will terminate
State is ‘ad hoc’ and can help if same error happens in multiple places
Range of 1 – 127

33. Sample Error Message
RaisError(‘Operation cannot be completed because field %s cannot be null’,1,1,’fieldname’)

34. Transactions Provides method for canceling an operation
Can restore rows, columns to original state in event of error or business logic failure
Use when changes will either be committed or discarded in entirety

35. ACID
Atomicity: All of the changes will be accepted or none of the changes will be accepted
Consistency: Data is either in its original or changed state
Isolation: If multiple transactions occur, data is never available in an intermediate state
Durability: Once finished, all changes are complete and changes can only be done by another transaction/unit of work

36. Using A Transaction Begin Tran: Identifies the start
Commit Tran: Write changes
Rollback Tran: Cancel changes
Be sure to issue a Commit or Rollback
Connection Stays Open Until Transaction is terminated

37. Locking & Concurrency
Locking allows a transaction to ensure that it can rollback
Prevents other operations from changing that data
Concurrency refers to multiple actions running against database at the same time
What happens if you want to change data I’m working with?

38. Sample Locking Levels Database Table Extent (memory)
Page (subset of extent)
Key
Row

39. Cursors Processing based on each row Declare @Cursor Cursor
not set operations
Cursor
= Cursor For (select statement)
Fetch Next into (variables matching field list in select)

40. Using a Cursor
Cursor = Cursor For (Select FirstName, LastName From Students) While = 0 Begin Fetch Next Into + ‘ End

41. @@Fetch_Status 0 success; -1 failed (read record outside recordset);
-2 missing record (eg. someone else deleted)