SELECT
  COUNT(b.`id_z`) as cnt, SUM(b.`sumd`) as sumd, post.`outsum` as post, 
SUM(`sumd`) -IFNULL( post.`outsum`, 0 ) AS debet
 	FROM zayavka b
LEFT JOIN (
  SELECT SUM(`postup`) as outsum, `id_z`
FROM `credit`
) post ON post.`id_z` = b.`id_z`

SELECT
  COUNT(b.`id_z`) as cnt, 
  SUM(b.`sumd`) as sum_d
FROM 
  zayavka b
WHERE  
  b.`id_z`  IN
  ( 
   SELECT `id_z` FROM `credit` WHERE credit.`id_z`=b.`id_z`
   HAVING(SUM(b.`sumd`)-IFNULL(SUM(credit.`postup`),0)=0)
  )

HAVING( SUM(b.`sumd`)=IFNULL(SUM(credit.`postup`),0) )

SELECT
  COUNT(b.`id_z`) as cnt, 
  SUM(b.`sumd`) as sum_d
FROM 
  zayavka b
WHERE  
  b.`id_z`  IN
  ( 
   SELECT `id_z` FROM `credit` WHERE credit.`id_z`=b.`id_z`
   HAVING( b.`sumd`=IFNULL(SUM(credit.`postup`),0) )
  )