Query Language

The Evolution of Database Systems

What is a database? In real meaning a database is nothing more than gathering of information that exists over a long phase of time, often many years.

Early Database Management Systems

The first commercial database management systems appeared in the late 1960's. These systems evolved from file systems, which offer some of item (3) above; file systems accumulate data over a long phase of time, and they allow the storage of huge amounts of data.

Corporate Records

Many early applications concerned corporate records, such as a record of each sale, information about accounts payable and receivable, or information about employees their names, addresses, salary, benefit options, tax status, and so on.

Overview of Query Processing

The great majority of connections with the DBMS follow the path on the left side of following figure. A user or an application program starts some action that does not affect the diagram of the database

Extents

When an ODL class is part of the database being described, we need to differentiate the class definition itself from the set of objects of that class that exist at a given time. The difference is the same as that between a relation schema and a relation instance, even though both can

Information Integration Via Semistructured Data

Unlike the other models we have discussed, data in the semistructured model is self-describing; the schema is attached to the data itself. That is, each node (except the root) has an arc or arcs entering it, and the labels on these arcs tell what role the node

The Database Language SQL

The most frequently used relational DBMSs query and modify the database through a language called SQL (often pronounced sequel). SQL stands for "Structured Query Language". The portion of SQL that supports queries has capabilities very close to that of relational algebra, as

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