與國外約束

Question

繼教科書「果殼中的SQL」的SQL查詢它要求查詢

有電子郵件地址的員工，展示他們的年齡和性別。

在下面的數據庫：

CREATE TABLE EMPLOYEES(
    PID SMALLINT NOT NULL, 
    PRIMARY KEY(PID), 
    CONSTRAINT EMPLOYEES_FK FOREIGN KEY (PID) REFERENCES PERSONS (PID) 
); 

CREATE TABLE PERSONS(
PID SMALLINT NOT NULL, 
NAME VARCHAR(20) NOT NULL, 
GENDER CHAR(1) NOT NULL CHECK (GENDER IN ('M','F')), 
AGE SMALLINT NOT NULL CHECK (AGE BETWEEN 0 AND 120), 
BIRTHDATE DATE NOT NULL, 
PRIMARY KEY(PID) 
); 

CREATE TABLE HASE(
PID SMALLINT NOT NULL, 
EADDR VARCHAR(30) NOT NULL, 
PRIMARY KEY(PID,EADDR), 
CONSTRAINT HASE_FK FOREIGN KEY (PID) REFERENCES PERSONS (PID) 
); 

我很新的SQL查詢，所以我的嘗試是：

SELECT GENDER, AGE 
FROM PERSONS AND EMPLOYEES 
WHERE EXISTS HASE.EADDR 
; 

但我不認爲，即時通訊與國外得到正確的關係鍵。我意識到這是非常基本的，但我認爲一旦我看到查詢的方式，它會幫助我與其他人。

Answer 1

您可以使用EXISTS，讓您的結果

SELECT Gender, Age 
FROM Persons 
    JOIN Employees ON Persons.PID = Employees.PID 
WHERE Exists (SELECT PID 
       FROM Hase 
       WHERE Persons.PID = Hase.PID) 

您也可以使用IN，但EXISTS查詢會更快

SELECT Gender, Age 
FROM Persons 
    JOIN Employees ON Persons.PID = Employees.PID 
WHERE Persons.PID IN (SELECT PID 
         FROM Hase) 

Answer 2

SELECT DISTINCT 
    persons.NAME, 
    persons.GENDER, 
    persons.AGE 
FROM 
    persons INNER JOIN employees 
    ON persons.PID = employees.PID 
    INNER JOIN hase 
    ON persons.PID = hase.PID 

請參閱小提琴here。