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

If you don't know there exist any data in your table or not, you can use following query:

SELECT cons_value FROM table_name;

For an Example:

SELECT 1 FROM employee;

1. It will return a column which contains the total number of rows & all rows have the same constant value 1 (for this time it returns 1 for all rows);
2. If there is no row in your table it will return nothing.

So, we use this SQL query to know if there is any data in the table & the number of rows indicates how many rows exist in this table.