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One-to-Many Relationship Type: ◦ The relationship that associates one entity to more than one entity is called one-to-many relationship. ◦ Symbol: ◦ Example: parent–child relationship. For one parent there can be more than one child

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A collection of tools for describing the database (include data, relationships, sematics, constraints) at a level of abstraction  Levels of abstraction: ◦ Conceptual Data Model:  a high level description useful for requirements understanding

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Cơ sở dữ liệu - Chương 1: Lập kế hoạch và cài dặt SQL server 2012 và 2008 Nên dành server rieengcho SQ Để tăng khả anwng sẵn sàng đáp ứng dịch vụ của server - Tính kinh tế

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In­doubt transactions have a , but neither a , nor an log record. ■ The recovering site must determine the commit­abort status of such transactions by contacting other sites; this can slow and potentially block recovery. ■ Recovery algorithms can note lock information in the log. ● Instead of , write out ...

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Example of a cache coherency protocol for shared disk systems: ● Before reading/writing to a page, the page must be locked in shared/exclusive mode. ● On locking a page, the page must be read from disk ● Before unlocking a page, the page must be written to disk if it was modified. ■ More complex protocols with fewer disk reads/writes exist. ...

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Shared memory contains shared data ● Buffer pool ● Lock table ● Log buffer ● Cached query plans (reused if same query submitted again) ■ All database processes can access shared memory ■ To ensure that no two processes are accessing the same data structure at the same time, databases systems implement mutual exclusion using either ● Operat...

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Solution: use number of hyperlinks to a site as a measure of the popularity or prestige of the site ● Count only one hyperlink from each site (why? ­ see previous slide) ● Popularity measure is for site, not for individual page  But, most hyperlinks are to root of site  Also, concept of “site” difficult to define since a URL prefix like cs....

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Data cleansing ● E.g. correct mistakes in addresses (misspellings, zip code errors) ● Merge address lists from different sources and purge duplicates ■ How to propagate updates ● Warehouse schema may be a (materialized) view of schema from data sources ■ What data to summarize ● Raw data may be too large to store on­line ● Aggregate values ...

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Database buffers are generally implemented in virtual memory in spite of some drawbacks: ● When operating system needs to evict a page that has been modified, the page is written to swap space on disk. ● When database decides to write buffer page to disk, buffer page may be in swap space, and may have to be read from swap space on disk and ou...

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Allow data items to be of various sizes and define a hierarchy of data granularities, where the small granularities are nested within larger ones ■ Can be represented graphically as a tree (but don't confuse with treelocking protocol) ■ When a transaction locks a node in the tree explicitly, it implicitly locks all the node's descendents in th...

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