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RDBMS Servers are increasingly common in client/server systems. Such database servers are often referred to as "SQL servers" or "database engines."
Figure 1. RDBMS Servers
In an RDBMS client/server environment, as in other client/server environments, the client controls the user interface. An RDBMS client usually presents the user with screen-based forms for data entry and display. The client also validates the user's input before submitting it to the server.
The RDBMS server manages the database. It controls the organization of the data in the database, ensures that the data is secure and safe from corruption, and that transactions are executed properly. It also usually includes general management functions such as backup and recovery of data. The RDBMS server may also contain application-specific queries and data manipulation statements called stored procedures. (We discuss stored procedures in more detail later in this lesson.)
The language that a client uses to communicate with an RDBMS server is called Structured Query Language (SQL, pronounced "See-quel"). The SQL language allows a client to query a database: that is, to ask a question that does not involve changing the contents of the database. Alternatively, a client can use a SQL statement to update existing data, insert new data, or delete data. For example, a client might issue a SQL statement requesting the RDBMS to return all information from the customer table where the customer's surname is "Smith." The server processes this request, executes it, and returns only the requested information. The client might then issue a second SQL statement to change the customer's current balance to "0." The server processes this request as well, changing the customer's balance and returning a "success" response to the client.
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