What does "select 1 from" do?

The construction is usually used in "existence" checks

if exists(select 1 from customer_table where customer = 'xxx')

or

if exists(select * from customer_table where customer = 'xxx')

Both constructions are equivalent. In the past people said the select * was better because the query governor would then use the best indexed column. This has been proven not true.

It does what you ask, SELECT 1 FROM table will SELECT (return) a 1 for every row in that table, if there were 3 rows in the table you would get

1
1
1

Take a look at Count(*) vs Count(1) which may be the issue you were described.

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.