Percentage calculation from a certain point in time

Question

I am trying to calculate the pass rate from after the ID studied a particular UNIT.
So I want to get the rate AFTER they studied their PREP01 unit
So right now  the code below works fine but it considers all units.
ID 1 studied one UNIT in 2010 which is before PREP so I do not want to consider it.
Also studied FGT459 in 2011 SP 1 as well as their FIRST PREP unit in SP 1 so i DO NOT want to consider it. IF A UNIT HAS BEEN DONE AT THE SAME TIME AS THE PREP01 UNIT I WOULD LIKE TO IGNORE IT.
After the PREP unit i want to consider any unit which has the Status as PRESENT. so out of the 3 remaning units they passed 1 and failed 3 so the percentage pass is 33.3%
Right now my code gives me 66.7% coz i condier all units.
I would only like to consider after PREP has been studied (PRESENT) IF A UNIT HAS BEEN DONE AT THE SAME TIME AS THE PREP01 UNIT I WOULD LIKE TO IGNORE IT.
Thanks for any assistance.
The code I tried is:
select  d1.id, ROUND(avg(case PASS_OR_BETTER
              when 'Y' then 1
              else          0
              end ) * 100,1)  as Pass
FROM DAN_SPIL_GRADE_IDS D1, DAN_SPIL_GRADE D2
WHERE D1.GRADE = D2.GRADE
AND D1.STATUS = 'PRESENT'
group by d1.id
TABLE1
CREATE TABLE DAN_SPIL_GRADE_IDS
(ID     VARCHAR2(8),
UNIT    VARCHAR2(8),
STATUS VARCHAR2(8),
GRADE VARCHAR2(12),
YEAR VARCHAR2(12),
PERIOD VARCHAR2(12));
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','FGT458','PRESENT','HD','2010','SP4');
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','FGT459','PRESENT','HD','2011','SP1');
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','PREP01','PRESENT','P','2011','SP1');
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','OPT878','ABSENT' ,'F','2011','SP2');
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','FRE111','PRESENT','D','2011','SP3');
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','PREP02','PRESENT','F','2012','SP1');
INSERT INTO DAN_SPIL_GRADE_IDS (ID, UNIT,STATUS,GRADE,YEAR,PERIOD) VALUES ('1','PREP03','PRESENT','F','2013','SP4');
ID
UNIT
STATUS
GRADE
YEAR
PERIOD
1
FGT458
PRESENT
HD
2010
SP4
1
FGT459
PRESENT
HD
2011
SP1
1
PREP01
PRESENT
P
2011
SP1
1
OPT878
APSENT
F
2011
SP2
1
FRE111
PRESENT
D
2011
SP3
1
PREP02
PRESENT
F
2012
SP1
1
PREP03
PRESENT
F
2013
SP4
TABLE2
CREATE TABLE DAN_SPIL_GRADE
(GRADE     VARCHAR2(8),
PASS_OR_BETTER VARCHAR2(12));
INSERT INTO DAN_SPIL_GRADE (GRADE, PASS_OR_BETTER) VALUES ('HD','Y');
INSERT INTO DAN_SPIL_GRADE (GRADE, PASS_OR_BETTER) VALUES ('D','Y');
INSERT INTO DAN_SPIL_GRADE (GRADE, PASS_OR_BETTER) VALUES ('P','Y');
INSERT INTO DAN_SPIL_GRADE (GRADE, PASS_OR_BETTER) VALUES ('F','N');
GRADE
PASS OR BETTER
HD
Y
D
Y
P
Y
F
N
GIves:
ID
PASS RATE
1
66.7
Want
ID
PASS RATE
1
33.3

Answer 1

Thanks for you assistance
I found an example which has  me puzzled
using your code on a case such as
ID
UNIT
STATUS
GRADE
YEAR
PERIOD
2
SUS108
PRESENT
F
2013
SP1
2
PREP01
PRESENT
F
2012
SP1
2
SSH100
PRESENT
F
2012
SP3
2
COD125
PRESENT
F
2012
SP2
2
SOC134
PRESENT
D
2012
SP1
2
SGY110
PRESENT
F
2011
SP1
2
SOC105
PRESENT
F
2012
SP3
Where ID 2 studied 4 units after PREP01 in 2012 SP 1
STUDIED:
2
2012
SOC105
SP3
OTHER
F
2
2013
SUS108
SP1
OTHER
F
2
2012
SSH100
SP3
OTHER
F
2
2012
COD125
SP2
OTHER
F
Your code gives 20% when it should give 0 because after PREP01 2012 SP1 the ID Failed everything