Topics

Views Stored Procedures User Defined Functions Triggers

Views

A view is a virtual table that consists of columns from one or more tables

Implements a security mechanism

Complex queries can be stored in the form as a view, and data from the view can be extracted using simple queries

T-SQL ViewViewsCREATE VIEW [owner.]view_name[(column_name [, column_name]...)][WITH ENCRYPTION]AS select_statement [WITH CHECK OPTION]

SQL Server stores information on the view in the following system tables:

o SYSOBJECTS — stores the name of the view. o SYSCOLUMNS — stores the names of the columns defined in the view. o SYSDEPENDS — stores information on the view dependencies. o SYSCOMMENTS — stores the text of the view definition.

o Views can have up to 1024 columns.o WITH ENCRYPTION encrypts the text for the view in the SYSCOMMENTS tables

The restrictions imposed on views are as follows

A view can be created only in the current database.

The name of a view must not be the same as that of the base table they must follow the rules for identifiers.

A view can be created only if there is a SELECT permission on its base table.

A SELECT INTO statement cannot be used in view declaration statement.

The CREATE VIEW statement cannot be combined with other SQL statements in a single batch.

SCHEMABINDING – Binds views to underlying tables.

The view may have to be modified or dropped to remove dependency on table

• If a view is not created with schemabinding clause sp_refreshview should be run when underlying table changes.

WITH CHECK OPTION – is an optional clause on the

CREATE VIEW statement that specifies the level of checking to be done when inserting or updating data through a view. If the option is specified, every row that is inserted or updated through the view must conform to the definition of that view

Alter Views

Drop Views When a view is dropped, it has no effect on the underlying tables. Dropping a view removes its definition and all the permissions

assigned to it. However, dropping a table that references a view does not drop the

view automatically. You must drop it explicitly.

Rename View

You can rename a view without having to drop it. This ensures that the permissions on the view are not lost

Modifying Data using Views

A view may be derived from multiple underlying tables A single data modification statement that affected both the

underlying tables is not permitted. You cannot modify the following of Columns using a view:

Columns that are based on computed values. E.g. sum, avg

Columns that are based on row aggregate functions. E.g. group by, having

Columns based on built-in functions like numeric, string functions.

Optimizing performance using Views

Indexed Views You can significantly improve performance by creating a unique clustered

index on a view that involves complex processing of large quantities of data, such as aggregating or joining many rows

Aggregations can be precompiled and stored in the index to minimize expensive computations during query execution

Unique clustered index is created on the view, the view's result set is materialized immediately and persisted in physical storage in the database, saving the overhead of performing this costly operation at execution time.

When to Use Indexed Views Because indexed views are more complex to maintain than indexes on base

tables, you should use them only when the improved speed in retrieving the results outweighs the increased overhead of data modifications. Indexing views is not a good idea in a high-volume OLTP system. Indexed views work best when the data is relatively static, and you need to

process many rows or the view will be referenced by many queries.

Indexed Views in SQL Server 2005 SQL Server 2005 contains many improvements for indexed

views compared with SQL Server 2000. Scalar aggregates, including SUM and COUNT_BIG

without GROUP BY. Scalar expressions and user-defined functions (UDFs) Common Language Runtime (CLR) types.

User-defined types (UDTs) UDFs based on the CLR

Database Tuning Advisor - recommends indexed views in addition to recommending indexes on base tables, and

table and index partitioning strategies

Requirements for Indexed Views Set the ANSI_NULLS option to ON when you create the tables referenced by the

view Set the ANSI_NULLS and QUOTED_IDENTIFIER options to ON prior to creating the

view

The view must only reference base tables, not any other views Base tables referenced by the view must be in the same database as the view

and must have the same owner Create the view and any user-defined functions referenced in the view with the

SCHEMABINDING option. This means that the underlying tables or other database objects cannot be altered or dropped as long as the view or function exists.

Reference all table names and user-defined functions with two-part names only—for example, "dbo.Customers" for the Customers table.

Any functions used by the view must be deterministic, meaning that the function must always return the same result anytime it's called with the same set of input values.

A unique clustered index must be created before any other indexes can be created on the view.

Additional disk space will be required to hold the data defined by the indexed view.

The following Transact-SQL syntax elements are illegal

in an indexed view

The * syntax to specify all columns. Column names must be explicitly stated.

Repeated columns—for example, SELECT Col1, Col2, Col1 AS Col. However, you can re-use a column if it's part of a different expression—for example, SELECT Col1, AVG(Col1), Col1 + Col2 AS Total

Derived tables and sub queries ROWSET. UNION. OUTER JOINS OR SELF JOINS. TOP AND ORDER BY. DISTINCT. COUNT(*). USE COUNT_BIG(*) INSTEAD, which returns a big int data type is

allowed. The following aggregate functions: AVG, MAX, MIN, STDEV, STDEVP, VAR. The definition of indexed view must be deterministic

CREATE TABLE T(a int, b real, c as getdate(), d as a+b)CREATE VIEW VT WITH SCHEMABINDING AS SELECT a, b, c, d FROM dbo.TSELECT object_id('VT'), COLUMNPROPERTY(object_id('VT'),'d','IsPrecise')

Examples

TSQL Stored Procedures Precompiled execution. SQL Server compiles each stored

procedure once and then reutilizes the execution plan. This results in tremendous performance boosts when stored procedures are called repeatedly.

Reduced client/server traffic. Stored procedures can reduce long SQL queries to a single line that is transmitted over the wire hence reduce client server traffic.

Efficient reuse of code and programming abstraction. Enhanced security controls. You can grant users permission

to execute a stored procedure.

Create / Alter Syntax

Rename Stored Procedure

Drop Procedure

Execute Stored Procedure

EXECUTE procedure_name

Parameterized Procedures

Error Handling in Stored Procedure

@@ERROR - This function is used to implement error handling code. It contains the error ID produced by the last SQL statement executed during a client’s connection. When a statement executes successfully, @@ERROR contains 0. To determine if a statement executes successfully, an IF statement is used to check the value of the function immediately after the target statement executes. It is imperative that @@ERROR be checked immediately after the target statement, because its value is reset when the next statement executes successfully

RAISERROR- The RAISERROR statement is used to produce an ad hoc error message or to retrieve a custom message that is stored in the sysmessages table.

Try..Catch Block

Implements error handling for Transact-SQL that is similar to the exception handling in the programming languages. A group of Transact-SQL statements can be enclosed in a TRY block. If an error occurs in the TRY block, control is

passed to another group of statements that is enclosed in a CATCH block. TRY…CATCH constructs can be nested. Either a TRY block or a CATCH block can

contain nested TRY…CATCH constructs. A TRY block must be immediately followed by an associated CATCH block.

Including any other statements between the END TRY and BEGIN CATCH

statements generates a syntax error. A TRY…CATCH construct cannot span multiple blocks of Transact-SQL

statements. For example, a TRY…CATCH construct cannot span two BEGIN…END blocks of Transact-SQL statements

In the scope of a CATCH block, the following system functions can be used to obtain information about the error that caused the CATCH block to be executed:

ERROR_NUMBER() returns the number of the error.

ERROR_SEVERITY() returns the severity.

ERROR_STATE() returns the error state number.

ERROR_PROCEDURE() returns the name of the stored procedure or trigger where the error occurred.

ERROR_LINE() returns the line number inside the routine that caused the error.

ERROR_MESSAGE() returns the complete text of the error message. The text includes the values supplied for any substitutable parameters, such as lengths, object names, or times.

User defined functions (UDF) Acts like a function in programming language. Can be

parameterized and called any number of times. Faster execution, Reduces network traffic. The ability for a function to act like a table (for Inline

table and Multi-statement table functions) gives developers the ability to break out complex logic into

shorter and shorter code blocks. Three types of UDFs

Scalar UDFs Inline Table valued UDFs Multi-statement table valued UDFs

Scalar UDFs

UDF returns a scalar data type. Text, ntext, image, timestamp are not supported.

Inline Table Valued UDFs

An Inline Table-Value user-defined function returns a table data type. Its an alternative to a view as the user-defined function can pass parameters into a T-SQL select command and in essence provide us with a parameterized, non-updateable view of the underlying tables

Multi-Statement Table valued UDFs A Multi-Statement Table-Value user-defined function

returns a table and is also an exceptional alternative to a view.

The ability to pass parameters into a T-SQL select command or a group of them gives us the capability to create a parameterized, non-updateable view of the data in the underlying tables.

Within the create function command you must define the table structure that is being returned.

After creating this type of user-defined function, you can use it in the FROM clause of a T-SQL command unlike the behaviour found when using a stored procedure which can also return record sets.

Limitations of UDFs

UDF Prohibit Usage of Non-Deterministic Built-in Functions. However it is allowed in SQL Server 2008.

UDF cannot Call Stored Procedure UDF have only access to Extended Stored Procedure.

UDFs cannot make use of dynamic SQL or temporary tables within the code. Table variables are allowed though.

UDF can not Return XML. UDF does not support SET options. UDF does not Support Error Handling

TRY/CATCH,RAISEERROR or @@ERROR are not allowed in UDFs.

UDF is allowed to modify the physical state of a database using INSERT, UPDATE or DELETE statements.

UDF can be called through a SQL statement without using the EXECUTE statement.

A UDF (any of the three variations - scalar, inline or multi-statement) cannot be used to return multiple result sets.

Triggers in SQL 2005 A trigger is a database object that is attached to a table. The main difference between a trigger and a stored

procedure is that the former is attached to a table and is only fired when an INSERT, UPDATE or DELETE occurs

Guards against malicious inserts and updates. Three types of Triggers in SQL 2005

Instead of Triggers After Triggers Data Definition Language Triggers

DML triggers use the deleted and inserted logical (conceptual) tables.

Triggers can allow cross table references, however check constraints allow column level constraints.

SQL Server 2000 provides four different ways to determine the affects of the DML statements.

The INSERTED and DELETED tables, popularly known as MAGIC TABLES update () columns_updated()

Magic Table does not contain the information about the columns of the data-type text, ntext, or image. Attempting to access these columns will cause an error.

update() function is used to find whether a particular column has been updated or not. This function is generally used for data checks. Returns a Boolean value.

Columns_Update() function returns a varbinary data type representation of the columns updated. This function return a hexadecimal values from which we can determine which columns in the table have been updated. COLUMNS_UPDATED tests for UPDATE or INSERT actions performed on

multiple columns To test for UPDATE or INSERT attempts on one column, use UPDATE().

AFTER Triggers Triggers that run after an update, insert, or delete can be

used in several ways: Triggers can update, insert, or delete data in the same

or other tables. This is useful to maintain relationships between data or to keep audit trail information.

Triggers can check data against values of data in the rest of the table or in other tables.

Triggers can use user-defined functions to activate non-database operations. This is useful, for example, for issuing alerts or updating information outside the database.

Can be specified only on tables not on views. AFTER trigger is a trigger that gets executed automatically

before the transaction is committed or rolled back. settriggerorder priority can set for AFTER triggers .

A table can have several AFTER triggers for each of the three triggering actions i.e., INSERT, DELETE and UPDATE

If a table has multiple AFTER triggers, then you can specify which trigger should be executed first and which trigger should be executed last using the stored procedure sp_settriggerorder

Like stored procedures and views, triggers can also be encrypted. The trigger definition is then stored in an unreadable form. Once encrypted, the definition of the trigger cannot be decrypted and cannot be viewed by anyone, including the owner of the trigger or the system administrator.

INSTEAD OF Triggers INSTEAD OF triggers facilitates updating Views.

A view or table can have only one INSTEAD OF trigger for each INSERT,

UPDATE and DELETE events

DDL Triggers in 2005

DDL triggers are fired on DDL events like Create, Alter, Drop.

schema_name cannot be specified for DDL or logon triggers. ALL

Indicates that all triggers defined at the scope of the ON clause are disabled

DATABASE For a DDL trigger, indicates that trigger_name was created

or modified to execute with database scope ALL SERVER

For a DDL trigger, indicates that trigger_name was created or modified to execute with server scope.

Why Triggers…?

If the database is de-normalized and requires an automated way to update redundant data contained in multiple tables

If customized messages and complex error handling are required

If a value in one table must be validated against a non-identical value in another table.

Triggers are a powerful tool that can be used to enforce the business rules automatically when the data is modified. Triggers can also be used to maintain the data integrity. But they are not to maintain data integrity. Triggers should be used to maintain the data integrity only if you are unable to enforce the data integrity using CONSTRAINTS, RULES and DEFAULTS.

Triggers cannot be created on the temporary tables.

DISABLE/ [ENABLE] TRIGGER Trigger_Name

ON ALL SERVER; DISABLE TRIGGER Person.uAddress ON

Person.Address; DISABLE TRIGGER safety ON DATABASE ; DROP TRIGGER Trigger_Name ON ALL

SERVER; DISABLE Trigger ALL ON ALL SERVER; Like stored procedures triggers can also

be encrypted. Triggers can be nested up to 32 levels.

More on Triggers

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