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Primary and Foreign Keys
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One important use of indexes is on referential constraints within a table. If you recall from 3, a referential constraint is where you ve indicated that through the use of a key, certain actions are constrained depending on what data exists. To give a quick example of a referential constraint, say you have a customer who owns banking products. A referential constraint would prevent the customer s record from being deleted while those products existed. SQL Server does not automatically create indexes on your foreign keys. However, as the foreign key column values need to be identified by SQL Server when joining to the parent table, it is almost always recommended that an index be created on the columns of the foreign key.
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Finding Specific Records
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Ideal candidates for indexes are columns that allow SQL Server to quickly identify the appropriate rows. In 8, we ll meet the WHERE clause of a query. This clause lists certain columns in your table and is used to limit the number of rows returned from a query. The columns used in the WHERE clause of your most common queries make excellent choices for an index. So, for example, if you wanted to find a customer s order for a specific order number, an index based on customer_id and order_number would be perfect, as all the information needed to locate a requested row in the table would be contained in the index. If finding specific records is going to make up part of the way the application works, then do look at this scenario as an area for an index to be created.
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Using Covering Indexes
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As mentioned earlier, when you insert or update a record, any data in a column that is included in an index is stored not only in the table, but also in the indexes for nonclustered indexes.
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CHAPTER 6 CREATING INDEXES AND DATABASE DIAGRAMMING
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From finding an entry in an index, SQL Server then moves to the table to locate and retrieve the record. However, if the necessary information is held within the index, then there is no need to go to the table and retrieve the record, providing much speedier data access. For example, consider the ShareDetails.Shares table in the ApressFinancial database. Suppose that you wanted to find out the description, current price, and ticker ID of a share. If an index was placed on the ShareId column, knowing that this is an identifier column and therefore unique, you would ask SQL Server to find a record using the ID supplied. It would then take the details from the index of where the data is located and move to that data area. If, however, there was an index with all of the columns defined, then SQL Server will be able to retrieve the description ticker and price details in the index action. It will not be necessary to move to the data area. This is called a covered index, since the index covers every column in the table for data retrieval.
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Looking for a Range of Information
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An index can be just as useful for finding one record as it can be for searching for a range of records. For example, say you wish to find a list of cities in Florida with names between Orlando and St. Petersburg in alphabetical order. You could put an index on the city name, and SQL Server would go to the index location of Orlando and then read forward from there an index row at a time, until it reached the item after St. Petersburg, where it would then stop. Because SQL Server knows that an index is on this column and that the data will be sorted by city name, this makes it ideal for building an index on a city name column. It should be noted that SQL Server indexes are not useful when attempting to search for characters embedded in a body of text. For example, suppose you want to find every author in a publisher s database whose last name contains the letters ab . This type of query does not provide a means of determining where in the index tree to start and stop searching for appropriate values. The only way SQL Server can determine which rows are valid for this query is to examine every row within the table. Depending on the amount of data within the table, this can be a very slow process. If you have a requirement to perform this sort of wildcard text searching, you should take a look at the SQL Server full-text feature, as this will provide better performance for such queries.
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