Select without a FROM clause in Oracle
SqlOracleSql Problem Overview
in SQL Server is possible to execute a SELECT, without reference to a table; something like:
Select 1.2 +3, 'my dummy string'
As Oracle does not allow a SELECT without a FROM, I use the dual table for this type of operation; something like:
Select 1,2+3, 'my dummy string' FROM DUAL
There is a better way of doing this type of query? it is good practice to use the dual table?
Sql Solutions
Solution 1 - Sql
No, in Oracle there is no SELECT without FROM.
Using the dual table is a good practice.
dual is an in-memory table. If you don't select DUMMY from it, it uses a special access path (FAST DUAL) which requires no I/O.
Once upon a time, dual had two records (hence the name) and was intended to serve as a dummy recordset to duplicate records being joined with.
Now it has but one record, but you can still generate an arbitrary number of rows with it:
SELECT level
FROM dual
CONNECT BY
level <= 100
MySQL also supports dual (as well as the fromless syntax).
Solution 2 - Sql
don't forget that most of the times you don't actually need to use SELECT.
Instead of:
SELECT sysdate INTO l_date FROM dual;
SELECT CASE WHEN i = j THEN 0 ELSE 1 END INTO l_foo FROM dual;
...
you can use
l_date := sysdate;
l_foo := CASE WHEN i = j THEN 0 ELSE 1 END;
...
Solution 3 - Sql
> it is good practice to use the dual > table
Yes, the dual table is usually used for this exact purpose. It's pretty standard in Oracle when you have no table to select from.
Solution 4 - Sql
Yes, the dual table is the usual way to do this in Oracle. As a matter of fact, it was introduced just for this.
The main advantage of DUAL is that the optimizer in Oracle knows it is special, so queries using it can be faster than if you used a single-row table you made yourself. Other than that, there's nothing special about it.
Solution 5 - Sql
I think you gotta use dual. it is used when you need to run SQL that does not have a table name. I can't say I use it much other than in SQL scripts to echo out the date something is ran or something stupid like that.
Solution 6 - Sql
Actually, SQL Server's implementation is non-standard. The SQL-92 Standard (Section 7.9) requires a FROM clause in a SELECT statement. DUAL is Oracle's way of providing a table to select from to get a scalar row.