Thursday, March 7, 2013

SQL Questions and Answers - Part 3

What is a view?
A view can be thought of as either a virtual table or a stored query. The data accessible through a view is not stored in the database as a distinct object. What is stored in the database is a SELECT statement. The result set of the SELECT statement forms the virtual table returned by the view. ( msdn.microsoft.com )

What is an Index?
A Microsoft® SQL Server™ index is a structure associated with a table or view that speeds retrieval of rows from the table or view. An index contains keys built from one or more columns in the table or view. These keys are stored in a structure that allows SQL Server to find the row or rows associated with the key values quickly and efficiently. ( msdn.microsoft.com )

What are all the different types of indexes?These are different types of indexes in Microsoft SQL Server 2008R2:
  1. Clustered: A clustered index sorts and stores the data rows of the table or view in order based on the clustered index key. The clustered index is implemented as a B-tree index structure that supports fast retrieval of the rows, based on their clustered index key values.
  2. Nonclustered: A nonclustered index can be defined on a table or view with a clustered index or on a heap. Each index row in the nonclustered index contains the nonclustered key value and a row locator. This locator points to the data row in the clustered index or heap having the key value. The rows in the index are stored in the order of the index key values, but the data rows are not guaranteed to be in any particular order unless a clustered index is created on the table.
  3. Unique: A unique index ensures that the index key contains no duplicate values and therefore every row in the table or view is in some way unique. Both clustered and nonclustered indexes can be unique.
  4. Index with included columns: A nonclustered index that is extended to include nonkey columns in addition to the key columns.
  5. Full-text: A special type of token-based functional index that is built and maintained by the Microsoft Full-Text Engine for SQL Server. It provides efficient support for sophisticated word searches in character string data.
  6. Spatial: A spatial index provides the ability to perform certain operations more efficiently on spatial objects (spatial data) in a column of the geometry data type. The spatial index reduces the number of objects on which relatively costly spatial operations need to be applied.
  7. Filtered: An optimized nonclustered index, especially suited to cover queries that select from a well-defined subset of data. It uses a filter predicate to index a portion of rows in the table. A well-designed filtered index can improve query performance, reduce index maintenance costs, and reduce index storage costs compared with full-table indexes.
  8. XML: A shredded, and persisted, representation of the XML binary large objects (BLOBs) in the xml data type column.
Read more at msdn.microsoft.com

Related posts:
SQL Questions and Answers – Part 3
SQL Questions and Answers – Part 2
SQL Questions and Answers – Part 1

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