问题
Which one is better for performance
IF EXISTS(Select null from table)
or
IF EXISTS(Select 1 from table)
?
回答1:
Both perform the same, because the SELECT clause in the EXISTS is never evaluated. You can test using:
... EXISTS(SELECT 1/0 FROM TABLE)
That should trigger a divide by zero error, but won't.
I personally prefer using NULL because it's obvious that nothing is referenced in the table, so it's more visible to others. Selecting a value, like the INT number 1 in the second example, can lead to assumptions about what is happening if not familiar with the EXISTS clause.
来源：https://stackoverflow.com/questions/4876166/performance-comparation-between-sql-select-null-and-select-1