how optimize this mysql query?

i have written this mysql query:

SELECT * , 1 AS haschild
       FROM table2
       WHERE parentid = '0'
             AND pid IN ( SELECT parentid FROM table2 )
UNION 
SELECT * , 0
       FROM table2
       WHERE parentid = '0'
             AND pid NO开发者_运维百科T IN ( SELECT parentid FROM table2 )
ORDER BY pid

but i think it is so silly query !! hope you can get my meaning of this query and guide me to write a better one.

thanks.

   SELECT t1.*,
          IF(t2.parentid IS NULL, 0, 1) AS haschild
     FROM table2 t1
LEFT JOIN table2 t2 ON t1.pid = t2.parentid
    WHERE t1.parentid = 0
 ORDER BY t1.pid

Notes:

1. Create index on parentid field
2. If parentid is a number - don't put quotes around

Try :

SELECT * , CASE WHEN pid IN ( SELECT parentid FROM table2 ) THEN 1 ELSE 0 END AS haschild
       FROM table2
       WHERE parentid = '0'
ORDER BY pid