select 1 from TABLE_NAME含义及用法

The construction is usually used in “existance” 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.