An Oracle database may include sample schemas, which are a set of interlinked schemas that enable Oracle documentation and Oracle instructional materials to illustrate common database tasks.

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The hr sample schema contains information about employees, departments and locations, work histories, and so on. The following illustration depicts an entity-relationship diagram of the tables in hr. Most examples in this manual use objects from this schema.

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Figure 2-3 HR Schema
Description of “Figure 2-3 HR Schema”

See also

Oracle Database Sample Schemas to learn how to install the sample schemas

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Overview of Tables

A table is the basic unit of data organization in an Oracle database.

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A table describes an entity, which is something of significance about which information must be recorded. For example, an employee could be an entity.

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Oracle Database tables fall into the following basic categories:

• Relational tablesRelational tables have simple columns and are the most common table type. Example 2-1 shows a CREATE TABLE statement for a relational table.
• Object tablesThe columns correspond to the top-level attributes of an object type. See “Overview of Object Tables”.

You can create a relational table with the following organizational characteristics:

• A heap-organized table does not store rows in any particular order. The CREATE TABLE statement creates a heap-organized table by default.
• An index-organized table orders rows according to the primary key values. For some applications, index-organized tables enhance performance and use disk space more efficiently. See “Overview of Index-Organized Tables”.
• An external table is a read-only table whose metadata is stored in the database but whose data is stored outside the database. See “Overview of External Tables”.

A table is either permanent or temporary. A permanent table definition and data persist across sessions. A temporary table definition persists in the same way as a permanent table definition, but the data exists only for the duration of a transaction or session. Temporary tables are useful in applications where a result set must be held temporarily, perhaps because the result is constructed by running multiple operations.

This topic contains the following topics:

• Columns
• Rows
• Example: CREATE TABLE and ALTER TABLE Statements
• Oracle Data Types
• Integrity Constraints
• Table Storage
• Table Compression