Introduction To Oracle Sql

Introduction to Oracle SQL •

Relational database tables are built on a strong mathematical foundation called relational theory. In this theory, relations or tables are operated on by a formal language called the relational algebra.

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SQL is a commercial interpretation of the relational algebraic constructs. Three concepts from relational theory encompass the capability of the SELECT statement: projection, selection and joining. o

Projection – refers to the restriction of attributes (columns) selected from a relation or table.

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Selection – refers to the restriction of the tuples or rows selected from a relation (table).

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Joining – refers to the interaction of tables with each other in a query.

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The Oracle Server stores information about all tables in a special set of relational tables called the data dictionary.

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The data dictionary stores definitions of database objects in a centralized, ordered, and structured format. The definition of a table includes information like table name, table owner, details about the columns that comprise it, and its physical storage size on disk. This information is also referred to as metadata.

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The structural metadata of a table may be obtained by querying the database for the list of columns that comprise it using the DESCRIBE command. The general syntax of the command is: DESCRIBE <SCHEMA>.tablename