Sometimes in Select queries you can see a mythic "dual" table.
What the hell is it? Let's see what is inside:
```sql
slect *
from dual
```
Result:
```
dummy
--------
X
```
This table contains one column (`dummy`) and one row (with `X` value),
but it's not important at all, because the `dual` table is used when
some "static" results are required.
The thing here is that Oracle **requires a table name** in `select
from` queries, even if selected data doesn't depend on tables used in
a query.
Because the `dual` table has only one row, all constants are returned
only once.
Here are a few examples:
Get constant values:
```sql
select 23
from dual
```
```sql
select 12 + 42
from dual
```
In the example below the value for the parameter `pvalue` comes from
outside, and the result of the query depends on it only, not on any
rows in a database:
```sql
select decode(:pvalue, 1, 'True', 'False')
from dual
```
Get a list of numbers (here we utilized the `level` pseudocolum):
```sql
select level lvl
from dual
connect by level < 11;
```
