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Types of Databases
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Relational database servers vary in the way they store information and in the way they allow multiple users to access that information simultaneously. Delphi provides support for two types of relational database server:

- Remote database servers reside on a separate machine. Sometimes, the data from a remote database server does not even reside on a single machine, but is distributed over several servers. Although remote database servers vary in the way they store information, they provide a common logical interface to clients. This common interface is Structured Query Language (SQL). Because you access them using SQL, they are sometimes called SQL servers. (Another name is Remote Database Management system, or RDBMS.) In addition to the common commands that make up SQL, most remote database servers support a unique "dialect" of SQL. Examples of SQL servers include InterBase, Oracle, Sybase, Informix, Microsoft SQL server, and DB2.
- Local databases reside on your local drive or on a local area network. They often have proprietary APIs for accessing the data. When they are shared by several users, they use file-based locking mechanisms. Because of this, they are sometimes called file-based databases. Examples of local databases include Paradox, dBASE, FoxPro, and Access.

Applications that use local databases are called single-tiered applications because the application and the database share a single file system. Applications that use remote database servers are called two-tiered applications or multi-tiered applications because the application and the database operate on independent systems (or tiers).

Choosing the type of database to use depends on several factors. For example, your data may already be stored in an existing database. If you are creating the database tables your application uses, you may want to consider the following questions:

- How many users will be sharing these tables? Remote database servers are designed for access by several users at the same time. They provide support for multiple users through a mechanism called transactions. Some local databases (such as Local InterBase) also provide transaction support, but many only provide file-based locking mechanisms, and some (such as client dataset files) provide no multi-user support at all.
- How much data will the tables hold? Remote database servers can hold more data than local databases. Some remote database servers are designed for warehousing large quantities of data while others are optimized for other criteria (such as fast updates).
- What type of performance (speed) do you require from the database? Local databases are usually faster than remote database servers because they reside on the same system as the database application. Different remote database servers are optimized to support different types of operations, so you may want to consider performance when choosing a remote database server.
- What type of support will be available for database administration? Local databases require less support than remote database servers. Typically, they are less expensive to operate because they do not require separately installed servers or expensive site licenses.

See Also

Database Security

Transactions

Referential Integrity

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