Tài liệu Công Nghệ Thông Tin

For a set X of attributes, we call the closure of X (with respect to a set of functional dependencies F), noted X+, the maximum set of attributes such that XX+ (as a consequence of F) Consider the relation scheme R(A,B,C,D) with functional dependencies {A}{C} and {B}{D}.  {A}+ = {A,C}  {B}+ = {B,D}  {C}+={C}  {D}+={D}  {A,B}+ = {A,...

22 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 981 | Lượt tải: 0

Example: The M:N relationship type WORKS_ON from the ER diagram is mapped by creating a relation WORKS_ON in the relational database schema. ◦ The primary keys of the PROJECT and EMPLOYEE relations are included as foreign keys in WORKS_ON and renamed PNO and ESSN, respectively. ◦ Attribute HOURS in WORKS_ON represents the HOURS attribute of ...

36 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 681 | Lượt tải: 0

Query languages: Allow manipulation and retrieval of data from a database.  Relational model supports simple, powerful QLs: ◦ Strong formal foundation based on logic. ◦ Allows for much optimization.  Query Languages != programming languages! ◦ QLs not intended to be used for complex calculations. ◦ QLs support easy, efficient access to ...

47 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 972 | Lượt tải: 0

In many cases an entity type has numerous subgroupings of its entities that are meaningful and need to be represented explicitly because of their significance to the database application.  Ex: EMPLOYEE may be further grouped into: ◦ SECRETARY, ENGINEER, TECHNICIAN,  Based on the EMPLOYEE’s Job ◦ MANAGER  EMPLOYEEs who are managers ◦ SAL...

39 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 726 | Lượt tải: 0

Cơ sở dữ liệu - Chương 2: Nhập xuất dữ liệu Exporting anh importing data Package là đói ngjcoots lõi trong SQL server SSIC dùng để chuyển dữ liệu từ nguồn đến đích và trình tự xử lý dữ liệu

71 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 729 | Lượt tải: 0

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

34 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 805 | Lượt tải: 0

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

24 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 700 | Lượt tải: 0

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

42 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 815 | Lượt tải: 0

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 ...

93 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 821 | Lượt tải: 0

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. ...

43 trang | Chia sẻ: huyhoang44 | Ngày: 16/03/2020 | Lượt xem: 806 | Lượt tải: 0