Question

Possible Duplicate:
What is the dual table in Oracle?

I have always wondered how does the query like:

select sysdate
from dual;

or

select 8
from dual;

do work.

As I’m used to select statement selects column/s value from a given table. But in the examples
mentioned above table dual DOESN’T have sysdate or just ‘8’ columns….

Could someone explain me how it works?

Answer 1

You don’t have to select columns. You can select expressions, which can by columns, constant values, function calls, calculations…. So you could select this from an employee table

select 
  e.nr,  -- column
  upper(e.name) as name, -- Function call to change the name to uppercase.
  e.age + 5 as age, -- expression with calculation with column value. 
  1  -- Just a value. Contains 1  for each row that the query returns.
from 
  employees e;

Note as name and as age. This is because the column name is lost once an expression contains more that just a column name. In that case, you can specify the name that you want the column to have in the query results. A similar trick is done for the table name itself. The alias e is just a shorthand. This is especially useful when using multiple tables (in a join). In this simple query, the table alias can be omitted altogether.

dual is just a special table that has 1 row exactly, and one column named dummy. It has many different uses which have been discussed before. From Oracle 23c onwards, from dual is optional, as it has been in MySQL for a while longer. You can simply write:

select
  1;