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
select count(*) from table where userid = :userid and rownum <= 1); -- If exists then 1 else 0
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;
Or you could do this:
select decode(max(USER_ID), null, 'FALSE', 'TRUE') BOOL_VAL
from USER_TABLE where USER_ID = [some USER_ID here]
Oracle RDBMS does not have boolean data type, you can only use boolean variables in PL/SQL.
If you simply want to return strings 'TRUE' and 'FALSE' you can do this..
SELECT 'TRUE' FROM DUAL WHERE EXISTS (SELECT 'x' FROM table WHERE user_id = 'id')
UNION
SELECT 'FALSE' FROM DUAL WHERE NOT EXISTS (SELECT 'x' FROM table WHERE user_id = 'id')
I like @DCookie's query though.
There is no Boolean type in Oracle SQL. You will need to return a 1 or 0, or some such and act accordingly:
SELECT CASE WHEN MAX(user_id) IS NULL THEN 'NO' ELSE 'YES' END User_exists
FROM user_id_table
WHERE user_id = 'some_user';