Oracle sql return true if exists question

Question

How do I check if a particular element exists in a table - how can I return true or false?

I have a table that has

• user_id
• user_password

Answer 1

In PL/SQL you can do this:

function user_exists (p_user_id users.user_id%type) return boolean
is
  l_count integer;
begin
  select count(*)
  into   l_count
  from   users
  where  user_id = p_user_id;

  return (l_count > 0);
end;

This would then be used in calling PL/SQL like this:

if user_exists('john') then
  dbms_output.put_Line('John exists');
end if;

NOTE: I used count(*) in the query in the knowledge that this will only return 1 or 0 in the case of a primary key search. If there could be more than one row then I would add "and rownum = 1" to the query to prevent unnecessarily counting many records just to find out if any exists:

function user_has_messages (p_user_id users.user_id%type) return boolean
is
  l_count integer;
begin
  select count(*)
  into   l_count
  from   messages
  where  user_id = p_user_id
  AND ROWNUM = 1;

  return (l_count > 0);
end;