Decode problem

Hi All ,

When i run this query at sql prompt it is running fine..

SQL> SELECT A.FOS_TYPE,B.DESCRIPTION,DECODE(FOS_TYPE,'D',
2 (select SUBSTR(CONCATENATED_SEGMENTS,1,4)
3 FROM FND_SHORTHANd_flex_ALIAses
4 where ALIAS_NAME = APS_CODE),'O',
5 (select SUBSTR(CONCATENATED_SEGMENTS,1,4)
6 FROM FND_SHORTHANd_flex_ALIAses
7 where ALIAS_NAME = APS_CODE),'PRAS')
8 FROM NT_DM_SMS_AR_TXN_V A ,FND_SHORTHANd_flex_ALIAses b
9 where A.APS_CODE = B.ALIAS_NAME
10 AND APS_CODE = 904 AND ROWNUM <= 1;

F
-
DESCRIPTION
----------------------------------------------------------------------------------------------------
DECO
----
D
Legacies Received - Bromley
FFAG


But when i use the same query in a pl/sql block or in for loop i get the following errors...i tried the whole yesterday...but i got the same errors..the errors are as follows...if any one has faced the same prob pls give the solution...thanks in advance


ORA-06550: line 3, column 1:
PLS-00103: Encountered the symbol "SELECT" when expecting one of the following:
begin function package pragma procedure subtype type use
cursor
form current
The symbol "begin" was substituted for "SELECT" to continue.
ORA-06550: line 4, column 1:
PLS-00103: Encountered the symbol "SELECT" when expecting one of the following:
( - + mod not null others
avg
count current exists max min prior sql stddev
ORA-06550: line 6, column 28:
PLS-00103: Encountered the symbol "," when expecting one of the following:
. ( ) * @ % & - + / at mod rem with and or
group having intersect minus start union where connect ||