Database Reference
In-Depth Information
Top-N queries allow restricting the number of rows to be returned
from a row set by using the ROWNUM pseudocolumn in the
WHERE clause.
Parallel queries are special queries designed to run faster in parallel
and are best executed on dual-CPU platforms, particularly with Ora-
cle Partitioning.
4.3.1
The DUAL Table
All DML statements create implicit cursors. Cursors are memory chunks
allocated for results of SQL statements. SELECT statements require a
source for an implicit cursor to operate on. Some types of SELECT state-
ments do not retrieve from any specific table. The DUAL table is a reposi-
tory for an expression result applied to a single value, acting as a temporary
repository for expression results, selected from the DUAL table.
The DUAL table can only be queried, never updated. The DUAL table
is owned by SYS but can be queried by any user. DUAL is useful when you
want to retrieve a constant or define a variable.
SELECT * FROM DUAL;
As you can see in Figure 4.14, the DUAL table contains a single col-
umn, a single row, and the value X in that single column. The column's
Figure 4.14
The DUAL Table
Is Available for
Special Use.
