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

To be slightly more specific, you would use this to do

SELECT 1 FROM MyUserTable WHERE user_id = 33487

instead of doing

SELECT * FROM MyUserTable WHERE user_id = 33487

because you don't care about looking at the results. Asking for the number 1 is very easy for the database (since it doesn't have to do any look-ups).