Create the following table in your database

   1: create table test_table

   2: ( 

   3: field_1 varchar2(10) not null,

   4: field_2 varchar2(10)

   5: );

   6:  

   7:  

   8: insert into test_table values(‘1′,‘1′);

   9:  

  10: insert into test_table values(‘2′, null);

Do

select count(*) from test_table

The number of records = 2

Do

select count(field_1) from test_table

The number of records = 2

Do

select count(field_2) from test_table

The number of records = 1. This is because it has NULL value in the 2nd record

Same for

select count(distinct field_2) from test_table

NULL value is excluded.

Kexi is an integrated data management application. It can be used for creating database schemas, inserting data, performing queries, and processing data. Forms can be created to provide a custom interface to your data. All database objects - tables, queries and forms - are stored in the database, making it easy to share data and design.

Kexi is considered as a long awaited Open Source competitor for Microsoft Access, FileMaker and Oracle Forms. Its development is motivated by the lack of Rapid Application Development RAD) tools for database systems that are sufficiently powerful, inexpensive, open standards driven and portable across many operating systems and hardware platforms.

Kexi is Free/Libre/Open-Source Software. As a real member of the KDE and KOffice projects, Kexi integrates fluently into both. It is designed to be fully usable also without KDE on Linux/Unix, Mac OS X (with Fink) and MS Windows platforms.

From the documentation,

The management of data in telecommunications system has many aspects whereof some, but not all, are addressed by traditional commercial DBMSs (Data Base Management Systems). In particular the very high level of fault tolerance which is required in many nonstop systems, combined with requirements on the DBMS to run in the same address space as the application, have led us to implement a brand new DBMS. called Mnesia. Mnesia is implemented in, and very tightly connected to, the programming language Erlang and it provides the functionality that is necessary for the implementation of fault tolerant telecommunications systems. Mnesia is a multiuser Distributed DBMS specially made for industrial telecommunications applications written in the symbolic programming language Erlang, which is also the intended target language. Mnesia tries to address all of the data management issues required for typical telecommunications systems and it has a number of features that are not normally found in traditional databases.
In telecommunications applications there are different needs from the features provided by traditional DBMSs. The applications now implemented in the Erlang language need a mixture of a broad range of features, which generally are not satisfied by traditional DBMSs. Mnesia is designed with requirements like the following in mind:

1. Fast real-time key/value lookup
2. Complicated non real-time queries mainly for operation and maintenance
3. Distributed data due to distributed applications
4. High fault tolerance
5. Dynamic re-configuration
6. Complex objects

What sets Mnesia apart from most other DBMSs is that it is designed with the typical data management problems of telecommunications applications in mind. Hence Mnesia combines many concepts found in traditional databases, such as transactions and queries with concepts found in data management systems for telecommunications applications, such as very fast real-time operations, configurable degree of fault tolerance (by means of replication) and the ability to reconfigure the system without stopping or suspending it. Mnesia is also interesting due to its tight coupling to the programming language Erlang, thus almost turning Erlang into a database programming language. This has many benefits, the foremost is that the impedance mismatch between data format used by the DBMS and data format used by the programming language, which is used to manipulate the data, completely disappears.

DatabaseAnswers.org has a vast collection of free data models for different domains. It can serve as a starting point for our application database design. You can then customize the database design to your specific requirements.

As quoted from the owner

My intention is to provide a wide range of ‘Kick Start’ Models that anyone can use as a starting-point, and could extend cleanly and logically, with appropriate reference to the Business Rules.
It is not my intention to provide Models that can be used off-the-shelf to meet the requirements of a large commercial organisation.
After all, that is one of the things I do for a living !!!
None of the Models is the complete and final solution in its area, but any of them can be added to easily and quickly to meet a specific requirement. The logic in each Model is correct and contains the minimum Entities for the area being modeled. It is easy to create a complex Model by combining these Kick-Start Models because they all follow the same design approach and standards.

Here are the top 20 models

1. Libraries and Books
2. Inventory Control for Retail Stores
3. Hotel Reservations
4. Video Rentals
5. School Management
6. Clients and Fees
7. CD Collections
8. Customers and Invoices
9. Payroll
10. Apartment Rentals
11. Customers and Services
12. ERP
13. Car Sales
14. Customers and Addresses
15. Driving Schools
16. Health and Fitness Clubs
17. Hospital Admissions
18. Inventory of Files in Boxes
19. Sports Clubs
20. Airline Reservations

As quoted, Power Architect is a user-friendly data modeling tool created by data warehouse designers, and has many unique features geared specifically for the data warehouse architect. It allows users to reverse-engineer existing databases, perform data profiling on source databases, and auto-generate ETL metadata. Plus, the Power*Architect has the ability to take snapshots of database structures, allowing users to design DW data models while working offline.

Features of Power Architect

• Accesses source databases via JDBC
• Connects to multiple source databases concurrently
• Compares data models & database structures and identifies discrepancies
• Drag-and-drop source tables & columns into the playpen
• Remembers the origin of each column
• Generates source-to-target visual Mapping Reports
• Forward-engineers to PostgreSQL, Oracle, MS SQL Server & more
• Saves source data structure snapshots in the project, so you can work remotely
• All project data is stored in an easy-to-parse XML format
• BSD licensed

Try out both and decide which one suits your needs..

I used SQuirreL SQL in almost all my projects. It supports a wide range of databases.

• Axion Java RDBMS.
• Daffodil (One$DB)
• Apache Derby (formerly Cloudscape) Using SQuirreL SQL with Derby (by Susan Cline)
• Fujitsu Siemens SESAM/SQL-Server with the SESAM/SQL JDBC driver.
• Firebird with the JayBird JCA/JDBC Driver.
• FrontBase
• HSQL.
• Hypersonic SQL.
• H2
• IBM DB2 for Linux, OS/400 and Windows.
• Informix.
• CA Ingres 2.6 SP2 with EDBC driver v1.8 (probably works with Caribou Lake driver too).
• InstantDB.
• Interbase with the Interclient JDBC driver.
• Mckoi SQL.
• Microsoft Access with the JDBC/ODBC bridge.
• Microsoft SQL Server with the Microsoft, WebLogic and jTDS JDBC drivers.
• Mimer SQL
• MySQL with the MM MySQL JDBC driver.
• Oracle 8i, 9i and 10g with the Oracle thin and OCI drivers.
• Pointbase.
• PostgreSQL 7.1.3 and higher..
• Progress OpenEdge 10.1x.
• SAPDB.
• Sybase with the JConnect JDBC driver. There are currently some issues with Sybase JConnect 5.5 and SQuirreL 1.1 although JConnect 4.5 seems to work.
• Sunopsis XML Driver (JDBC Edition).
• Teradata Warehouse
• ThinkSQL RDBMS.

SQuirreL SQL can be extended easily using plugin. Mogwai ER Designer NG has a plugin for it and I found it useful using them together. Now you easily using an ER designer in SQuirreL SQL easily.

PolePosition is a benchmark test suite to compare database engines and object-relational mapping technology. It has done the benchmarking for the followings

• db4o - the open source object database for Java and .NET
• Hibernate - relational persistence for idiomatic Java
• MySQL - the world’s most popular open source database
• McKoi - an open source Java SQL database system
• JavaDB - Sun’s supported distribution of Apache Derby
• HSQLDB - lightweight 100% Java SQL database engine
• SQLite - a small C library that implements a self-contained, embeddable, zero-configuration SQL database engine.

Few things to note from the tests, as described in the website,

1. HSQLDB is performing good as most processing is done in memory, but if you want to use it in complex database application, you have to test yourself
2. Use of OR mapping tools like Hibernate or JDO has negative impact on performance. Use it if you have powerful hardware. Well, Java always demands higher processing power, that is what we all knew :). However, O-R mappers and object databases make life so much easier and productive.
3. db4o provides best performance for deep inheritance hierarchies and tree structures
4. MySQL is worth looking at compared to other proprietary products. Well I like this one.