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Oracle® Database Administrator's Guide
11g Release 1 (11.1)

Part Number B28310-01
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About Tables

Tables are the basic unit of data storage in an Oracle Database. Data is stored in rows and columns. You define a table with a table name, such as employees, and a set of columns. You give each column a column name, such as employee_id, last_name, and job_id; a datatype, such as VARCHAR2, DATE, or NUMBER; and a width. The width can be predetermined by the datatype, as in DATE. If columns are of the NUMBER datatype, define precision and scale instead of width. A row is a collection of column information corresponding to a single record.

You can specify rules for each column of a table. These rules are called integrity constraints. One example is a NOT NULL integrity constraint. This constraint forces the column to contain a value in every row.

You can also invoke transparent data encryption to encrypt data before storing it. If users attempt to circumvent the database access control mechanisms by looking inside Oracle datafiles directly with operating system tools, encryption prevents these users from viewing sensitive data.

Tables can also include virtual columns. A virtual column is like any other table column, except that its value is derived by evaluating an expression. The expression can include columns from the same table, constants, SQL functions, and user-defined PL/SQL functions. You cannot explicitly write to a virtual column.

After you create a table, you insert rows of data using SQL statements or using an Oracle bulk load utility. Table data can then be queried, deleted, or updated using SQL.

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