sql server 2008 - What does "select 1 from" do?

Question

I read some articles but really didn't understand what does select 1 from do? Someone says "you should use select 1 instead of select *". Here is an example table:

cust_id     cust_name       cust_address

1000000001  Village Toys    Mapl
1000000002  Kids Place      South
1000000003  Fun4All         Sunny
1000000004  Fun4All         Riverside
1000000005  The Toy Store   53rd

What will the result be when I write select 1 from customer_table what does this statement do?

Answer 1

select 1 from table

will return a column of 1's for every row in the table. You could use it with a where statement to check whether you have an entry for a given key, as in:

if exists(select 1 from table where some_column = 'some_value')

What your friend was probably saying is instead of making bulk selects with select * from table, you should specify the columns that you need precisely, for two reasons:

1) performance & you might retrieve more data than you actually need.

2) the query's user may rely on the order of columns. If your table gets updated, the client will receive columns in a different order than expected.