Having count of 1

Question

I would like to count how many ID have only had 1 Enrolment by year:
I am assuming you would have to use having count = 1 or something like that.
So I have the following example below
ID 1 has 2 enrolments so dont count
ID 2 has only 1 enrolment so count
ID 3 has 2 enrolments so dont count
ID 4 has 1 enrolment so count
ID 5 has 2 enrolments so dont count
ID 6 has 1 so count
ID 7 and 8 have one each so count.
Thanks
CREATE TABLE DAN_PRU
(ID     VARCHAR2(8),
UNIT    VARCHAR2(8),
YEAR VARCHAR2(8))
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (1,'POP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (1,'DOP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (2,'KUIP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (3,'DIP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (3,'YUP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (4,'SII','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (5,'DYY','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (5,'LIW','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (6,'DFG','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (7,'DWQ','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (8,'DFR','2013');GIVES:
ID     UNIT     YEAR
1     POP     2013
1     DOP     2013
2     KUIP     2013
3     DIP     2013
3     YUP     2013
4     SII     2013
5     DYY     2012
5     LIW     2012
6     DFG     2012
7     DWQ     2011
8     DFR     2011WANT
ENROLMENTS     YEAR
2            2013
1            2012
2            2011
using oracle 10

Answer 1

Hi,
Interesting problem!
Chloe_19 wrote:
I would like to count how many ID have only had 1 Enrolment by year:
I am assuming you would have to use having count = 1 or something like that.That's an obvious first step. What makes this such an interesting problem is how you get from there to the final answer.
What is the final answer? Do you want just one row, containing just one number (5 in this case, since there are 5 ids (2, 4, 6, 7 and 8) that have only 1 enrollment?
So I have the following example below
ID 1 has 2 enrolments so dont count
ID 2 has only 1 enrolment so count
ID 3 has 2 enrolments so dont count
ID 4 has 1 enrolment so count
ID 5 has 2 enrolments so dont count
ID 6 has 1 so count
ID 7 and 8 have one each so count.
Thanks
CREATE TABLE DAN_PRU
(ID     VARCHAR2(8),
UNIT    VARCHAR2(8),
YEAR VARCHAR2(8)) Why is year a VARCHAR2? Why is it 8 characters long?
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (1,'POP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (1,'DOP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (2,'KUIP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (3,'DIP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (3,'YUP','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (4,'SII','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (5,'DYY','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (5,'LIW','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (6,'DFG','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (7,'DWQ','2013');
INSERT INTO DAN_PRU (ID, UNIT, YEAR) VALUES (8,'DFR','2013');
GIVES:How does the data above give the results below.
For example, it looks like '2013' is the only year in the data. How do you get '2011' and '2012' in the results?
Can the same id have multiple years? What results would you want in that case?
ID     UNIT     YEAR
1     POP     2013
1     DOP     2013
2     KUIP     2013
3     DIP     2013
3     YUP     2013
4     SII     2013
5     DYY     2012
5     LIW     2012
6     DFG     2012
7     DWQ     2011
8     DFR     2011WANTAre you saying the results below are what you want?
ENROLMENTS     YEAR
2            2013
1            2012
2            2011
using oracle 10Can you get a result set that contains one row per id? (Or for each id/year combination; it's unclar what you want.)
Can you restrict that result set to contain only the ids with only 1 enrollment?
If you had a table like the previous result set, how would you solve the problem? Remember, any result set can be treated like a table, using a WITH clause.