SQL Query with a count condition - sql

Question

Here is my table structure:
CREATE TABLE CITY(
CITY_ID NUMBER(3) CONSTRAINT CITY_ID_PK PRIMARY KEY,
CITY_NAME VARCHAR2(20) CONSTRAINT CITY_NAME_NN NOT NULL);
CREATE TABLE PILOT(
PILOT_ID NUMBER(3) CONSTRAINT PILOT_ID_PK PRIMARY KEY,
LAST_NAME VARCHAR2(20) CONSTRAINT LAST_NAME_NN NOT NULL,
FIRST_NAME VARCHAR2(20) CONSTRAINT FIRST_NAME_NN NOT NULL,
CITY_ID NUMBER(3) CONSTRAINT CITY_ID_FK REFERENCES CITY(CITY_ID),
SALARY NUMBER(7,2) CONSTRAINT SALARY_CK CHECK (SALARY >= 5000 AND SALARY <= 7000));
CREATE TABLE PLANE(
PLA_ID NUMBER(2) CONSTRAINT PLANE_ID_PK PRIMARY KEY,
PLA_DESC VARCHAR2(20) CONSTRAINT PLANE_DESC_NN NOT NULL,
MAX_PASSENGER NUMBER(3),
CITY_ID NUMBER(3) CONSTRAINT PLANE_CITY_ID_FK REFERENCES CITY(CITY_ID),
CONSTRAINT MAX_PASSENGER_CK CHECK (MAX_PASSENGER <= 500));
CREATE TABLE FLIGHT(
FLIGHT_ID NUMBER(3) CONSTRAINT FLIGHT_ID_PK PRIMARY KEY,
PILOT_ID NUMBER(3) CONSTRAINT FLIGHT_PILOT_ID_FK REFERENCES PILOT(PILOT_ID),
PLA_ID NUMBER(2) CONSTRAINT FLIGHT_PLA_ID_FK REFERENCES PLANE(PLA_ID),
CITY_DEP NUMBER(3) CONSTRAINT FLIGHT_CITY_DEP_FK REFERENCES CITY(CITY_ID),
CITY_ARR NUMBER(3) CONSTRAINT FLIGHT_CITY_ARR_FK REFERENCES CITY(CITY_ID),
DEP_DATE DATE,
DEP_TIME NUMBER(4),
ARR_TIME NUMBER(4),
CONSTRAINT ARR_TIME_CK CHECK (ARR_TIME > DEP_TIME));
The question I have in this lab is to display pilots (ID and Name) who perform two or more flights out of Montreal (It is required that I use the city name in the query and not the ID)
Here is what I have come up with so far:
SELECT PILOT_ID, LAST_NAME, FIRST_NAME
FROM PILOT
JOIN FLIGHT USING (PILOT_ID)
WHERE CITY_DEP=(SELECT CITY_ID
FROM CITY
WHERE CITY_NAME='MONTREAL')
Obviously this gets me part of the answer, but it is not displaying exactly the information I need which is simply the pilots who make this fight >= 2 times.

Answer 1

You can use FETCH ROWS
SELECT PILOT_ID, LAST_NAME, FIRST_NAME
FROM PILOT
JOIN FLIGHT USING (PILOT_ID)
WHERE CITY_DEP=(SELECT CITY_ID
FROM CITY
WHERE CITY_NAME='MONTREAL')
FETCH FIRST 2 ROWS ONLY

Answer 2

Edited with new information on data structure
Understanding your goal
I believe I understand your goal to be querying pilot level data for pilots who have departed from Montreal at least twice in one day.
Query Solution
If my assumptions are true, I believe you can meet your needs by doing something similar to this:
CREATE GLOBAL TEMPORARY TABLE flight_per_day ON COMMIT PRESERVE ROWS AS
SELECT
p.pilot_id,
f.dep_date,
COUNT(CASE WHEN c.city_name = 'MONTREAL' THEN 1 ELSE NULL END) as
montreal_cnt
FROM flights f
LEFT JOIN pilot p ON p.pilot_id = f.pilot_id
LEFT JOIN city c on f.city_dep = c.city_id
GROUP BY 1, 2;
SELECT
p.pilot_id,
p.first_name,
p.last_name
FROM flight_per_day fp
LEFT JOIN pilot p ON p.pilot_id = fp.pilot_id
WHERE fp.montreal_cnt>=2
or without a temp table you could do
SELECT
p.pilot_id,
p.first_name,
p.last_name
FROM
(SELECT
p.pilot_id,
f.dep_date,
-- Find the total number of flights (COUNT) where (CASE WHEN) a flight departs from Montreal (THEN) count it otherwise (ELSE) ignore it (NULL)
COUNT(CASE WHEN c.city_name = 'MONTREAL' THEN 1 ELSE NULL END) as
montreal_cnt
FROM flights f
-- Join in pilot table to get the counts by pilot_id
LEFT JOIN pilot p ON p.pilot_id = f.pilot_id
-- Join in city table to get city_name instead of city_id
LEFT JOIN city c on f.city_dep = c.city_id
GROUP BY 1, 2) fp
LEFT JOIN pilot p ON p.pilot_id = fp.pilot_id
-- Only give me the data for pilots who have flown out of Montreal at least twice in one day
WHERE fp.montreal_cnt>=2

Answer 3

For each pilot, you need to count how many flights that pilot has from Montreal, and then retrieve the pilots that have 2 or more flights. This is a job for GROUP BY and HAVING.
SELECT PILOT_ID, LAST_NAME, FIRST_NAME
FROM PILOT
JOIN FLIGHT USING (PILOT_ID)
JOIN CITY ON (CITY_DEP = CITY_ID)
WHERE CITY_NAME='MONTREAL'
GROUP BY PILOT_ID, LAST_NAME, FIRST_NAME
HAVING COUNT(*) >= 2;