Stored procedures

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

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

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

EXECUTE
 EXEC(cute) statement
OR
 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)

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

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)

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

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

Sample Output Parameter
CREATE PROC upFindStudentID
@First varchar(25),
@Last varchar(35),
@SID char(9) OUTPUT
AS
SELECT @SID=SchoolID
FROM Students_T
Where @First=Firstname and
@Last=Lastname

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

Variables
 Create using DECLARE
 Need to start with ‘@’
 Can use SQL data types or custom data types

DECLARE @StudentName varchar(50)

Variable Assignment
 SET is usually used similar to procedural language
SET @Var=value

 SELECT is usually used when getting a value from a
query
SELECT @Var=Sum(PossiblePoints) FROM Assignments

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

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

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

Searched CASE
 No test expression
 Each WHEN has a boolean test

CASE
WHEN Points >= 90 THEN ‘A’
WHEN Points < 90 AND Extra > 0
THEN ‘A’
END

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)

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

@@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)

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.

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

@@Rowcount
 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

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

@@Error
 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

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

Raise Error
 Used to send information to calling program
 Syntax:
RaisError (Message string OR Message ID, Severity,
State)
 Severity – <14 information; 15-19 warning or user
can correct; 20+ fatal
 State – way to differentiate problems if needed;
typically use 1
 RAISERROR (50001,16,1)

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

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

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

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

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

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

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

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?

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

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

Using a Cursor
Declare @Students Cursor
Set @Cursor = Cursor For (Select FirstName, LastName From
Students)
Open @Students
While @@Fetch_Status = 0
Begin
Fetch Next From @Students Into @First, @Last
Print @First + ‘ ‘+ @Last
End
Close @Students
Deallocate @Students

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

Stored procedures

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Stored procedures