Question

这是我的表结构：

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));

我在本实验中遇到的问题是显示在蒙特利尔以外进行两次或更多次飞行的飞行员（ID和姓名）（要求我在查询中使用城市名称而不是ID）

这是我到目前为止提出的：

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')

显然，这使我成为了答案的一部分，但它并不能完全显示我需要的信息，而只是飞行员进行了2次以上的战斗。

Answer 1

您可以使用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

使用有关数据结构的新信息进行编辑

了解您的目标

我相信我理解您的目标是查询一天中至少两次离开蒙特利尔的飞行员的飞行员水平数据。

查询解决方案

如果我的假设是正确的，我相信您可以通过执行以下操作来满足您的需求：

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

或者没有临时表，您可以做

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

对于每个飞行员，您需要计算该飞行员从蒙特利尔出发的飞行次数，然后检索具有2个或更多航班的飞行员。这是GROUP BY和HAVING的工作。

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;

具有计数条件的SQL查询

3 个答案:

了解您的目标

查询解决方案