Eliminating duplicates from subquery...

Question

What is the best way to eliminate duplicates from a subquery:
SELECT dept_no, dept_name
FROM dept D
WHERE EXISTS (
SELECT 'X'
FROM emp E
WHERE E.dept_no = D.dept_no);
OR
SELECT dept_no, dept_name
FROM dept D
WHERE ( SELECT 'X'
FROM emp E
WHERE E.dept_no = D.dept_no AND ROWNUM < 2);
Thanks!

Answer 1

>
UPDATE TABLE1
SET COL1 = (
SELECT DISTINCT COL1
FROM TABLE2, TABLE3
WHERE TABLE2.ID = TABLE3.ID
You need to refine your example. At present you appear to be updating every row in table1 to the same value - but only if col1 (which could be from table2 or table3 - or might be accidental capture from table1) holds just one distinct value across the query; but it looks as if you're likely to get 'single row subquery returns more than one row[ as an error.
I guess you're trying to do something LIKE:
update t1
set t1.col1 = (
  select t2.col2
  from t2
  where  t2.colx = t1.coly
  and  exists (
    select null
    from t3
    where t3.id = t2.id
)The most efficient access path depends on how many rows will have to be examined in each table, and how many times you will have to jump to another table to find related data - and if your query is roughly the shape of this one, the optimizer may be able to transform it in a variety of ways to find an efficient access path.
As it stands, my example will be setting col1 to null whenever there is no match for coly in table t2 - and the optimizer would have to drive off t1 looking at every row to do this. Your requirement, and available predicates, indexes and constraints, may allow, or force, a completely different strategy.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk