Select without a FROM clause in Oracle

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).

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;
...

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.

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