Why do I have to select from the dual table?

Question

This works in the major Relation Database Management Systems most likely to appear on StackOverflow/dba.stackexchange, being SQL Server, MySQL, PostgreSQL and SQLite (WebSQL).

select 'abc' abc, 1 def;

It does not work on Oracle. Why do we need to select from DUAL in Oracle? Does the ISO/ANSI standard for SQL require a FROM clause for SELECT statements?

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Edit:

Per Bacon Bit's answer, it does seem required by the SQL standard.

So in reality, because the name DUAL is such a misnomer, if I were to create a table and name it ATOM or ONE, e.g. create table one (atom int); .. select 'abc' abc, 1 def FROM one; - Is there a performance penalty compared to SELECT .. FROM DUAL?

Answer 1

Strictly, yes, the FROM clause of a SELECT statement is not optional. The syntax for SQL-99 details the basic SELECT statment, and the FROM clause doesn't have any square brackets around it. That indicates the standard considers it non-optional:

SELECT [ DISTINCT | ALL ]
{Column expression [ AS name ]} [ ,... ] | *
FROM <Table reference> [ {,<Table reference>} ... ]
[ WHERE search condition ]
[ GROUP BY Columns [ HAVING condition ] ]
[ORDER BY {col_name | expr | position} [ASC | DESC],...]                                     
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[PROCEDURE procedure_name(argument_list)]
[INTO OUTFILE 'file_name' export_options |
 INTO DUMPFILE 'file_name' |
 INTO var_name [, var_name]]
[FOR UPDATE | LOCK IN SHARE MODE]

In actual use, programmers and DBAs often find it useful to do things other than manipulate data in tables or manipulate tables and data structures. This type of thing is largely beyond the scope of the SQL standard, which is concerned with the data features more than the nuts and bolts of specific implementations. Whether we want to run SELECT getdate() or SELECT 1 or SELECT DB_NAME() (or whatever your dialect prefers), we don't actually want data from a table.

Oracle chooses to solve the standard and implementation discrepancy using a dummy table with the following effective definition:

CREATE TABLE DUAL (
  DUMMY CHAR(1)
  )

INSERT INTO DUAL (DUMMY) VALUES ('X')

Other RDBMSes essentially assume that a dummy table is used if no FROM is specified.

The history of the DUAL table is on Wikipedia:

The DUAL table was created by Charles Weiss of Oracle corporation to provide a table for joining in internal views:

I created the DUAL table as an underlying object in the Oracle Data Dictionary. It was never meant to be seen itself, but instead used inside a view that was expected to be queried. The idea was that you could do a JOIN to the DUAL table and create two rows in the result for every one row in your table. Then, by using GROUP BY, the resulting join could be summarized to show the amount of storage for the DATA extent and for the INDEX extent(s). The name, DUAL, seemed apt for the process of creating a pair of rows from just one.

The original DUAL table had two rows in it (hence its name), but subsequently it only had one row.

Answer 2

The advantage to dual is the optimizer understands dual is a special one row, one column table (with varchar2 data type) -- when you use it in queries, it uses this knowledge when developing the plan.

Why do we need to select from dual in Oracle?

You can select from dual or from your own tables too, you can if you want.

For me, I'll stick with dual because I know dual exists. I know it has at least 1 and at most 1 row. I know the optimizer knows all about dual and does the most efficient thing for me. The optimizer understands dual is a magic, special 1 row table. It stopped on the select * because there is to be one row in there. So it's just the way it works.

Answer 3

The other two answers provide good background for my answer.

On Oracle databases, it is traditional and reliable. It will fail on other databases which don't have a DUAL table. It is not necessary that you use DUAL, but I would recommend you do.

Standards compliant databases will require a FROM clause specifying at least Table reference. If you have an ORDERS table the following replacement for the FROM DUAL clause would work:

FROM   orders
WHERE  rownum =1

Substitute any table or view you can select from and it will work. Substitute a table or view you can't select from and it will fail. DUAL is more reliable as barring a DBA breaking it, all users can select from it, and will get only one row in the result set. (It does get broken occasionally.)

I am unaware of a standards compliant verb for accessing data which is not in a table without including a Table reference. Given how little I access such data, I don't see such a need. Many of the cases I run into, can be better handled in different ways.