Variant  1 

SELECT DISTINCT a.name
FROM
  CUST a,
  PRIXOD b
WHERE
  a.id = b.cust_id
GROUP BY
  a.NAME
HAVING
  sum(b.msum) >  5000 

  0      SELECT STATEMENT Optimizer=CHOOSE
   1    0     FILTER
   2    1       SORT (GROUP BY) 
   3    2         NESTED LOOPS
   4    3           TABLE ACCESS (FULL) OF 'PRIXOD'
   5    3           TABLE ACCESS (BY INDEX ROWID) OF 'CUST'
   6    5             INDEX (UNIQUE SCAN) OF 'SYS_C002182' (UNIQUE)

Variant2

select a.name
from cust a,(select cust_id from prixod b group by cust_id having sum(msum)> 5000 ) b
where a.id=b.cust_id

  0      SELECT STATEMENT Optimizer=CHOOSE
   1    0     NESTED LOOPS
   2    1       VIEW
   3    2         FILTER
   4    3           SORT (GROUP BY) 
   5    4             TABLE ACCESS (FULL) OF 'PRIXOD'
   6    1       TABLE ACCESS (BY INDEX ROWID) OF 'CUST'
   7    6         INDEX (UNIQUE SCAN) OF 'SYS_C002182' (UNIQUE)

Variant  3 

select a.name from cust a
where exists
(select * from
(select cust_id from prixod b group by cust_id having sum(msum)> 5000 ) b
where a.id=b.cust_id
)

  0      SELECT STATEMENT Optimizer=CHOOSE
   1    0     FILTER
   2    1       TABLE ACCESS (FULL) OF 'CUST'
   3    1       VIEW
   4    3         FILTER
   5    4           SORT (GROUP BY) 
   6    5             TABLE ACCESS (FULL) OF 'PRIXOD'


Variant  4 

select a.name
from cust a
where exists
(
select cust_id from prixod b where a.id=b.cust_id group by cust_id having sum(msum)> 5000 
)


  0      SELECT STATEMENT Optimizer=CHOOSE
   1    0     FILTER
   2    1       TABLE ACCESS (FULL) OF 'CUST'
   3    1       FILTER
   4    3         SORT (GROUP BY) 
   5    4           TABLE ACCESS (FULL) OF 'PRIXOD'