This means that if we change the table schema (i.e add and/or remove columns) the changes will not be visible to the view – the view definition will not automatically change to support the table changes. The view definition will include all columns from the underlying table, dbo.T1 at the time of the view creation. SELECT * in the context of a View definition behaves differently then when used as a query element in a batch. Just a few basic guidelines on how to define SQL Views. Indexed views is Sql Server( similar but not the same as the materialised views in other db platforms) are special type of views that can have their result-set permanently stored on disk – more on indexed views can be found here. Views are not materialised, meaning that the rows produced by views are not stored permanently on disk. The main advantage of Views over other types of table expressions is their re-usability i.e derived queries and CTE have scope limited to a single statement. A view definition is stored as an Sql Server object along with objects such as user defined tables, triggers, functions, stored procedures etc. Views (sometimes referred to as virtual relations) are reusable table expressions. The derived table AUSCust is visible only to the outer query and the scope is limited to the sql statement. The following code represents a derived table called AUSCust. The scope of derived tables is always the outer query. The expressions are defined in the FROM clause of an outer query. The visibility of the table expressions may vary i.e views and ITVF are db objects visible on a database level, whereas they scope is always on an SQL statement level – table expressions cannot operate across different sql statements within a batch.īenefits of table expressions are not related to query execution performances but to the logical aspect of the code ! Derived Tablesĭerived tables are table expressions also known as sub-queries. They are virtual tables present only in RAM memory (they may be spilled to disk as a result of i.e memory pressure, size of a virtual table etc.). In general, table expressions are not materialised on the disk. ITVF (Inline Table Valued Functions aka parameterised views).Sql Server supports four types of table expressions Table ExpressionsĪ table expression is a named query expression that represents a relational table. In this post I will try to explain how CTE recursion works, where it sits within the group of table expressions available in Sql Server and a few case scenarios where the recursion shines. It is usually used to resolve hierarchies. A recursive CTE is a type of CTE that references itself. They represent one of several types of table expressions available in Sql Server. Common Table Expressions were introduced in SQL Server 2005.
0 Comments
Leave a Reply. |