Thư viện tài liệu, ebook tham khảo dành cho học sinh, sinh viên

Insertion Insert more column data than desired Must know student number and offering number to insert a new course Update Change multiple rows to change one fact Must change two rows to change student class of student S1 Deletion Deleting a row causes other facts to disappear Deleting enrollment of student S2 in offering O3 causes loss of i...

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Identify entity types and attributes Determine primary keys Add relationships Determine connections Determine relationship cardinalities Simplify relationships

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Identification dependency M-N relationships with attributes Self identifying relationships M-way relationships Equivalence between M-N and 1-M relationships

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The GROUP BY clause is used to group rows to compute group-statistics.  The syntax of GROUP BY command is: SELECT attribute name, aggregate function FROM table name GROUP BY attribute name;

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Uses the concepts of FDs, primary key  Definitions: ◦ Prime attribute - attribute that is member of the primary key K ◦ Full functional dependency - a FD Y  Z where removal of any attribute from Y means the FD does not hold any more

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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,...

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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 ...

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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 ...

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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...

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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

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