Counting in Oracle 11g - sql

Question

all day I am struggling with oracle exrcises and again I stuck. I need to select last names of boxers with their wins of each weight category.
So I have:
table "boxer" with columns: id, fname, lname, weight
table "fight" with two foreign keys from table boxer (id_boxer1 and id_boxer2) and with one column winner (if boxer1 won then winner will be number 1, if boxer2 won then winner will be number 2)
table "category_weight" with columns: id, min_weight, max_weight, name (of a category)
Example:
CREATE TABLE category_weight(
id INTEGER NOT NULL,
min_weight SMALLINT NOT NULL,
max_weight SMALLINT NOT NULL,
name VARCHAR2(20) NOT NULL
);
ALTER TABLE category_weight ADD CONSTRAINT category_weight_pk PRIMARY KEY ( id );
CREATE TABLE boxer(
id INTEGER NOT NULL,
fname VARCHAR2(20) NOT NULL,
lname VARCHAR2(20) NOT NULL,
weight INTEGER NOT NULL
);
ALTER TABLE boxer ADD CONSTRAINT boxer_pk PRIMARY KEY ( id );
CREATE TABLE fight(
id INTEGER NOT NULL,
winner SMALLINT NOT NULL,
id_category_weight INTEGER NOT NULL,
id_boxer1 INTEGER NOT NULL,
id_boxer2 INTEGER NOT NULL
);
ALTER TABLE fight ADD CONSTRAINT fight_pk PRIMARY KEY ( id );
ALTER TABLE fight
ADD CONSTRAINT boxer_fk FOREIGN KEY ( id_boxer1 )
REFERENCES boxer ( id );
ALTER TABLE fight
ADD CONSTRAINT boxer_fk2 FOREIGN KEY ( id_boxer2 )
REFERENCES boxer ( id );
ALTER TABLE fight
ADD CONSTRAINT categ_weight_fk FOREIGN KEY ( id_category_weight )
REFERENCES category_weight ( id );
INSERT INTO boxer
VALUES ('1', 'Johnny','Johnny','60');
INSERT INTO boxer
VALUES ('2', 'Anthonny','Anthonny','54');
INSERT INTO boxer
VALUES ('3', 'Anonimm','Anonimm','59');
INSERT INTO boxer
VALUES ('4', 'John','Johnowski','71');
INSERT INTO category_weight
VALUES ('1', '1','70','category1');
INSERT INTO category_weight
VALUES ('2', '71','100','category2');
INSERT INTO fight
VALUES ('1','1','1','1','2');
INSERT INTO fight
VALUES ('2','2','1','3','1');
Boxer with ID "1" won two fights in category1, so the result should be:
Lname Category Wins
Johnny category1 2

Answer 1

Here, try this:
SELECT b.lname,
cw.max_weight AS WEIGHT_CLASS,
COUNT(CASE WHEN f.winner = b.id THEN 1 ELSE NULL END) AS WINS
FROM boxer b
INNER JOIN fight f ON b.id = f.id_boxer1 OR b.id = f.id_boxer2
INNER JOIN category_weight cw ON f.id_category_weight = cw.id
GROUP BY b.lname, cw.max_weight