Chapter 1 Database Access from Client Applications

Query by Example (QBE) n No official standard n First pass at SQL n Interactive querying or updating n Usability hierarchy n objects n functions/expressions n macros n VBA n API

Building Queries using QBE n Visual image of the table is used for writing queries. n Result is a dynaset n not a base table n dynamic or virtual set n may or may not be updateable n Joins (relationships) based on relationship view or created in query

Joins n Inner (normal) join n instructors and sections they teach n Outer join n all classes and scheduled sections n Self join n students and their advisors

Nested Queries n Use first query to compute lists or statistics necessary for second query n which students have any status with MIS 320? n which students have no status with MIS 320? n which sections have two or more students (with any status)?

Access 97 Query Types n Select n SQL n Total:  group by, aggregate functions n Action: create, delete, update, append n Crosstab n Top(n)

Client Applications n Code generators n Switchboard: main menu form tied to special table n or, make your own n removing extraneous items

Reports n Banded report design n report header n page header n group header [repeatable] n detail n group footer n page footer n report footer

Access as a Front End n Access as the file server, uses built-in JET engine n ODBC with JET, translates queries from Access to server SQL n SQL pass-through, query must be written for server SQL n ODBCdirect, an extension to JET

Embedded SQL n SQL commands in 3GL programs n more flexible, accessible interface n improve performance n improve database security n Host program  precompiler  Source code and expanded SQL  compiler  Object code  linker/editor  Executable code

Visual Basic for Applications (VBA) n Benefits n complex functionality n error handling n faster execution n maintenance n OLE automation n more programmatic control n easier to read n Event-driven n event occurs n event detected n response generated

Chapter 2 Distributed Databases

Distributed Database n “A single logical database that is spread physically across computers in multiple locations that are connected by a data communications network.” n conditions that encourage use of distributed databases: n distribution and autonomy of business units n data sharing n data communications costs and reliability

Distributed Database Environments n Homogeneous n autonomous n non-autonomous n Heterogeneous n systems n full functionality n partial multi-database n gateways

Homogeneous Distributed Database Environment n Data distributed across all nodes n Same DBMS at each location n All data managed by the distributed DBMS n One global schema

DDBMS Objectives n Location transparency n Local autonomy n Synchronous or asynchronous n Advantages: n increased reliability n local control n modular growth n lower communications costs n faster response

Options for Distributing a Database n Data replication n Horizontal partitioning n Vertical partitioning n Combinations of the above

Data Replication n Advantages n reliability n fast response n less complicated integrity routines n node decoupling n reduced network traffic at prime time n Disadvantages n storage requirements n complexity and cost of updating

More on Data Replication n Schemes n snapshot replication n near real-time replication n pull replication n Replication favored when n process requests are read-only n data are relatively static

Partitioning (H or V) n Advantages n efficiency n local optimization n security n ease of querying n Disadvantages n inconsistent access speeds n backup vulnerability

Comparison of Strategies

Distributed DBMS n Functions n distributed data dictionary n retrieval and processing locations n translate from one DBMS to another n data management functions such as security n consistency across locations

DDBMS Architecture n Each site: n local DBMS n local database n distributed DBMS n distributed data repository n communications controller

DDBMS Objectives n Location transparency n as if all data located at a single node n Replication transparency n Failure transparency n detect … reconfigure … recover n transaction manager maintains log and concurrency control scheme n two-phase commit protocol n Concurrency transparency n timestamping