A Closer Look at Select in Java

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4 A Closer Look at Select
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single value attributes, it does not produce different results than using AND, but usually results in slower performance
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Selection with EVERY()
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SimpleDB enables you to query for multi-valued attributes based on matching a comparison against every existing value, rather than just a single value, which is the normal case To match a comparison for every attribute value, you use the EVERY() functionThe function is applied to the attribute name or the itemName() on the left side of the comparison operator For example:
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SELECT itemName() FROM bookmarks WHERE every(tag) LIKE '%net'
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The preceding query will not return items that have a tag value ending in net if other tag values with other endings are presentThe EVERY() function can be applied to any of the operators, but it does not make sense to use it with IS NULL or IS NOT NULL:
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SELECT * FROM students WHERE EVERY(`test-score`) BETWEEN '060' AND '075' SELECT * FROM students WHERE EVERY(`project-status`) IN('Completed','Excused')
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When every() is combined with the equals operator, it enables you to select items with exactly one value and no more:
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SELECT itemName() FROM bookmarks WHERE EVERY(tag) = 'net'
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Query Results with the Same Item Multiple Times
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One of the things to consider when constructing a query for multi-valued attributes is whether it is possible for more than one value to match the criteria Consider, again, the fictitious domain that contains tagged bookmarksA query to find all bookmarks with a tag that ends with net might look like this:
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SELECT itemName() FROM bookmarks WHERE tag LIKE '%net'
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Table 4-2 shows the sample data set for these examples
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Table 4-2 ItemName
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Bookmark0 Bookmark1 Bookmark2 Bookmark3
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Sample Data: Items with Multiple Tag Values Tag
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ajaxnet vbnet aspnet aspx jscriptnet jsonnet systemnet c#
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Multi-Valued Attribute Queries
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First, you can see that all four of these bookmarks have at least one tag ending in net The second thing to notice is that Bookmark0 and Bookmark2 each have two different tags that match the criteriaWhen SimpleDB processes this Select, it will return each of these items exactly once without duplicates:
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Bookmark0 Bookmark1 Bookmark2 Bookmark3
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The issue arises, however, if the Select contains a sort directive on the attribute with multiple matches Suppose the query is changed to read as follows:
SELECT itemName() FROM bookmarks WHERE tag LIKE '%net' ORDER BY tag
Now you can see that returning each matching bookmark exactly once is going to be a problem Looking at the sorted list of tag values, Bookmark0 has a match at the beginning with ajaxnet and the end with vbnet And this is exactly how SimpleDB responds to this query: with Bookmark0 at both the beginning and the end of the list:
Bookmark0 Bookmark1 Bookmark2 Bookmark3 Bookmark0
An additional point to notice is that Bookmark2 also has two tags that match the criteria but it is not returned twice in the resultsThe reason for this is that the two tags ( jscriptnet and jsonnet ) appear sequentially in the sorted list SimpleDB returns the same item as many times as necessary to satisfy the ORDER BY directive However, separate entries for the same item are never generated for sequential ORDER BY matches
Improving Query Performance
The SimpleDB Select query language does not have nearly the same capabilities as SQL and so is less powerful, but by the same token avoids a great deal of the complexity However, there is still quite a bit that you can do with it, and the opportunity certainly exists to create complex and inefficient queriesAchieving good query performance is important no matter what database or query language you use
Attribute Indexes
An index is a database structure that occupies extra storage space for each record in exchange for faster data retrieval In a typical relational database, you must create and manage each indexThis includes deciding which and how many columns to index for each table, as well as when to rebuild the indexes Performing the analysis on each index to determine if it is providing more benefit than cost can be time consuming