What do you put in a subquery's Select part when it's preceded by Exists?

Question

What do you put in a subquery's Select part when it's preceded by Exists?

Select *  
  From some_table  
 Where Exists (Select 1  
                From some_other_table  
               Where some_condition )

I usually use 1, I used to put * but realized it could add some useless overhead. What do you put? is there a more efficient way than putting 1 or any other dummy value?

Answer 1

I think the efficiency depends on your platform. In Oracle, SELECT * and SELECT 1 within an EXISTS clause generate identical explain plans, with identical memory costs. There is no difference. However, other platforms may vary.

As a matter of personal preference, I use

  SELECT *

Because SELECTing a specific field could mislead a reader into thinking that I care about that specific field, and it also lets me copy / paste that subquery out and run it unmodified, to look at the output.

However, an EXISTS clause in a SQL statement is a bit of a code smell, IMO. There are times when they are the best and clearest way to get what you want, but they can almost always be expressed as a join, which will be a lot easier for the database engine to optimize.

SELECT *
FROM SOME_TABLE ST
WHERE EXISTS(
  SELECT 1
  FROM SOME_OTHER_TABLE SOT
  WHERE SOT.KEY_VALUE1 = ST.KEY_VALUE1
    AND SOT.KEY_VALUE2 = ST.KEY_VALUE2
  ) 

Is logically identical to:

SELECT *
FROM
  SOME_TABLE ST
  INNER JOIN
  SOME_OTHER_TABLE SOT
  ON ST.KEY_VALUE1 = SOT.KEY_VALUE1
    AND ST.KEY_VALUE2 = SOT.KEY_VALUE2

Answer 2

I also use 1. I've seen some devs who use null. I think 1 is efficient compared to selecting from any field as the query won't have to get the actual value from the physical loc when it executes the select clause of the subquery.

Answer 3

Use:

WHERE EXISTS (SELECT NULL
                FROM some_other_table  
               WHERE ... )

EXISTS returns true if one or more of the specified criteria match - it doesn't matter if columns are actually returned in the SELECT clause. NULL just makes it explicit that there isn't a comparison while 1/etc could be a valid value previously used in an IN clause.