CHAPTER

th SQLNIGHT

Designing and Implementing Views

Sotiris Karras

Fivi Panopoulou

Oct 10, 2015

26

• Introduction to Views

• View options

• Updatable Views

• Indexed Views

Agenda

Views

A view is a virtual table whose contents are defined by a query

CREATE VIEW [ schema_name . ] view_name [ (column [ ,...n ] ) ] [ WITH <view_attribute> [ ,...n ] ] AS select_statement [ WITH CHECK OPTION ] [ ; ] <view_attribute> ::= {

[ ENCRYPTION ] [ SCHEMABINDING ] [ VIEW_METADATA ]

}

Views

• Query is eventually applied to the underlying object

• Views can be used for• Simplification and customization

• Security

• Backward compatibility

• The select statement cannot include• ORDER BY clause, unless it is used with TOP or OFFSET

• Reference to temporary table or variable

• INTO and OPTION keywords

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View Options

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• CHECK OPTION

• The CHECK OPTION in the SELECT statement of a

CREATE VIEW statement allows the user to specify rules

that any modifications against the view must be

compliant with.

• Any modifications that happen in the underlying tables

that violate the rules within the CHECK OPTION are not

visible in the view.

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• SCHEMABINDING

• The SCHEMABINDING option, binds the view to the

schema of the underlying tables.

• The SCHEMABINDING option can be used only with

objects within the same database and in the SELECT

statement the participating objects must be referenced

as (schema.object).

• Other views or tables cannot be altered or dropped if

they are participating in schemabinded views.

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• ENCRYPTION and VIEW_METADATA

• Encrypts the entries in sys.syscomments that contain the

text of the CREATE VIEW statement. Using WITH

ENCRYPTION prevents the view from being published

as part of SQL Server replication.

CHAPTER

DEMO

Views

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Updatable Views

• Views through which the data of the underlying tables

can be modified.

• Managing security privileges across multiple tables

easier.• User can modify underlying table through the view, f he haw the

corresponding permission on the view.

Updatable Views

• The modifications in an UPDATE, DELETE or INSERT statement can only

reference one underlying table at a time.

• The underlying columns to be modified, must be directly referenced by the

columns of the view. Cannot update columns that are derived by aggregate

functions or other computations.

• TOP clause is not present in the SELECT statement of the view.

• The columns being modified are not affected by GROUP BY, HAVING, or

DISTINCT clauses.

• Rule of thumb: The Database engine must be able to UNAMBIGUOUSLY trace

modifications from the view to the underlying table.

Restrictions

• The above restrictions can be avoided by creating an INSTEAD OF trigger on

the view.

• The trigger, explicitly specifies the actions to be taken if a data modification

statement is to be made on the view.

• By using an INSTEAD OF trigger, every view can be made updatable.

Bypassing the restrictions

CREATE TRIGGER <trigger_name>ON <view>INSTEAD OF{INSERT, UPDATE, DELETE}AS <SQL_code_here>;

CHAPTER

DEMO

Updatable Views

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Indexed Views

• Indexed View: a view with a unique clustered index on it

• Indexed views are used for performance tuning

• An indexed view can be updatable

• Query optimizer can use the index to speed up queries

• Additional non-clustered indexes can be created

• Indexed views affect insert and update operations on

base tables

Indexed Views

• Verify SET options for session and underlying tables

• Check that the view definition is deterministic

• Create the view with SCHEMABINDING

• Create a unique clustered index on the view

Creating an Indexed View

SET OPTIONS REQUIRED VALUE

ANSI_NULLS ON

ANSI_PADDING ON

ANSI_WARNINGS ON

ARITHABORT ON

CONCAT_NULL_YIEDS_NULL ON

NUMERIC_ROUNDABORT OFF

QUOTED IDENTIFIER ON

• All expressions in the select list, the WHERE and the

GROUP BY clause must be deterministic

• Deterministic expression: returns the same result every

time evaluated on the same input

• Precise expression: do not contain float expressions

• Float expression can only be non-key columns

• Only precise deterministic expressions can participate in

key columns.

Deterministic Views and Precise Columns

• If GROUP BY is present • the definition must contain COUNT_BIG(*)

• must not contain HAVING

• the key can reference only columns specified in the GROUP BY

clause

• CLR functions can appear in the select list, but cannot

be part of the key

• The view cannot reference objects in other databases

Restrictions

• The statement must not contain the following elements

Restrictions

COUNT DISTINCT * MIN, MAX

ORDER BY TOP OFFSET STDEV, STDEVP, VAR, VARP, AVG

OUTER joins Self-joins UNION,EXCEPT,INTERSECT CUBE, ROLLUP, GROUPING SETS

Subquery CTE Derived table table-valued functions

CLR user-defined aggregate

function

PIVOT, UNPIVOT APPLY text, ntext, image, XML, filestream

columns

Full-text predicates TABLESAMPLE Table variables ROWSET functions

SUM function on nullable

expression

Sparse column

sets

CHECKSUM_AGG OVER clause, which includes ranking

or aggregate window functions

… and some more restrictions and requirements https://msdn.microsoft.com/en-us/library/ms191432.aspx#Restrictions

CHAPTER

DEMO

Indexed Views

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