Thư viện tài liệu, ebook tổng hợp lớn nhất Việt Nam

Step 9: Mapping of Union Types (Categories). For mapping a category whose defining superclass have different keys, it is customary to specify a new key attribute, called a surrogate key, when creating a relation to correspond to the category. In the example below we can create a relation OWNER to correspond to the OWNER category and include any ...

31 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 1016 | Lượt tải: 0

Formal Definitions of EER Model (2) Subclass S of C is predicate defined if predicate p on attributes of C is used to specify membership in S; that is, S = C[p], where C[p] is the set of entities in C that satisfy p A subclass not defined by a predicate is called user-defined Attribute-defined specialization: if a predicate A = ci (where A is a...

55 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 1330 | Lượt tải: 0

The Domain Relational Calculus Another variation of relational calculus called the domain relational calculus, or simply, domain calculus is equivalent to tuple calculus and to relational algebra. The language called QBE (Query-By-Example) that is related to domain calculus was developed almost concurrently to SQL at IBM Research, Yorktown Heigh...

47 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 895 | Lượt tải: 0

UPDATE Used to modify attribute values of one or more selected tuples A WHERE-clause selects the tuples to be modified An additional SET-clause specifies the attributes to be modified and their new values Each command modifies tuples in the same relation Referential integrity should be enforced

76 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 1760 | Lượt tải: 0

Classification of DBMSs Based on the data model used: Traditional: Relational, Network, Hierarchical. Emerging: Object-oriented, Object-relational. Other classifications: Single-user (typically used with micro- computers) vs. multi-user (most DBMSs). Centralized (uses a single computer with one database) vs. distributed (uses multiple compute...

33 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 897 | Lượt tải: 0

Cost-estimation-based optimization If the cost of every execution plan can be estimated accurately, then the optimal plan can be eventually found. Two difficulties with cost-estimation based optimization:  There may be too many possible execution plans to enumerate.  It may be difficult to estimate the cost of each execution plan accurately. ...

70 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 997 | Lượt tải: 1

Updating tuples in a nested table: update table (select beers from manfs m where m.name = ‘Anheuser’) b set b.color = ‘blue’; First, you have to flatten the object table using the pseudo function table before you can do an update on the object table. Not quite obvious, because update essentially refers only to the object table beers and not ...

93 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 948 | Lượt tải: 0

Research Directions (2006-2010) (cont.) Advanced Information Systems: E-/M-Commerce SOA-Based Modern Information Systems Large Database Systems Web Information Systems Modern Information Retrieval Systems Stream Data Management* Bioinformatics*

51 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 946 | Lượt tải: 0

Summary Introduction to Database Security Issues Types of Security Threats to databases Database Security and DBA Access Protection, User Accounts, and Database Audits Discretionary Access Control Types of Discretionary Privileges Specifying Privileges Using Views Revoking Privileges Propagation of Privileges Using the GRANT OPTION An ex...

53 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 1020 | Lượt tải: 0

Additional Query Tuning Guidelines WHERE conditions may be rewritten to utilize the indexes on multiple columns. Example: SELECT REGION#, PROD_TYPE, MONTH, SALES FROM SALES_STATISTICS WHERE REGION# = 3 AND ((PRODUCT_TYPE BETWEEN 1 AND 3) OR (PRODUCT_TYPE BETWEEN 8 AND 10)); may use an index only on REGION# and search through all leaf pa...

24 trang | Chia sẻ: vutrong32 | Ngày: 19/10/2018 | Lượt xem: 959 | Lượt tải: 0