mysql - 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, what will it return?

Also, in what type of scenarios, can this be used?

Answer 1

select 1 from table will return the constant 1 for every row of the table. It's useful when you want to cheaply determine if record matches your where clause and/or join.