What does it mean by select 1 from table?

Question

I have seen many queries with something as follows.

Select 1  
From table

What does this 1 mean, how will it be executed and,

Answer 1

I see it is always used in SQL injection,such as:

www.urlxxxxx.com/xxxx.asp?id=99 union select 1,2,3,4,5,6,7,8,9 from database;

These numbers can be used to guess where the database exists and guess the column name of the database you specified.And the values of the tables.