Query a table and exit when first record found

Question

I want to query a table and just want to know if any rows matching my query criteria exist in the table. I just want to find one row, irrespective of order and stop the query right there. How do I do that?

Answer 1

The most efficient way would be either to use rownum = 1 as part of the condition in your second query, but, as written, both will return multiple rows. Your first will need to have an additional predicate and rownum = 1 in addition to the exists.
Assuming that the predicate can use an index, then the most efficient approach would be either:
SELECT 1 FROM table
WHERE <conditions> and
      rownum = 1or possibly:
SELECT 1 FROM dual
WHERE EXISTS (SELECT 1 FROM table
              WHERE <conditions>)Both will do a range scan on the applicable index, stopping when the find the first matching entry. To my mind, the first is clearer in intent.
To illustrate the error in your first query, consider:
SQL> SELECT * FROM t;
        ID DESCR
         1 One
         2 Two
         3 Three
         4 Four
         5 Five
         1 One
         2 Two
         3 Three
         4 Four
         5 Five
SQL> SELECT * FROM t
  2  WHERE id = 1 and
  3        rownum = 1;
        ID DESCR
         1 One
SQL> SELECT * FROM t
  2  WHERE EXISTS (SELECT 1 FROM t
  3                WHERE id = 1);
        ID DESCR
         1 One
         2 Two
         3 Three
         4 Four
         5 Five
         1 One
         2 Two
         3 Three
         4 Four
         5 FiveJohn
Edited by: John Spencer on Oct 2, 2009 12:06 PM
Added queries from t