Why Use Oracle's DUAL Table in SELECT Queries Without a FROM Clause?

Leveraging Oracle's Dual Table for Select Queries Without a FROM Clause

In contrast to SQL Server, Oracle enforces the presence of a FROM clause in SELECT queries. To overcome this restriction, practitioners often resort to utilizing the dual table.

Best Practice: Using Dual

The article advocates for the continued use of the dual table in such scenarios. Dual serves as an efficient in-memory table that supports a specialized access path known as FAST DUAL, eliminating I/O operations.

Historical Context

Originally designed with two records, the dual table facilitated record duplication in joins. Today, while it contains a single record, it can generate multiple rows using constructs like:

SELECT  level
FROM    dual
CONNECT BY
        level <= 100

MySQL Compatibility

It is noteworthy that MySQL also supports dual and the fromless syntax. These features provide flexibility and consistency across different database systems.

Additional Considerations

Despite its advantages, the dual table may not always be the ideal solution for all use cases. It is essential to consider factors such as performance and scalability requirements when crafting queries.

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