如何在 SQL 中使用 COUNT 和 JOIN？

Question

我需要找到飲酒者，飲酒者為所有飲酒者評分的總次數。 我還需要包括尚未對飲品評分的飲酒者。 我需要使用 JOIN 或 LEFT/RIGHT OUTER JOIN 操作。

到目前為止，這是我的代碼：

SELECT LIKES.DRINKER, COUNT(LIKES.RATING)
FROM LIKES JOIN ORDERS.DRINK = LIKES.DRINK
GROUP BY LIKES.DRINKER; 

這是我要加入的表：

CREATE TABLE LIKES( /* Drinkers like drinks */
DRINKER     VARCHAR(30) NOT NULL,   /* Drinker name */
DRINK       VARCHAR(30) NOT NULL,   /* Drink name   */
RATING      DECIMAL(1)  NOT NULL,   /* Rating of the drink  */
    CONSTRAINT LIKES_PKEY PRIMARY KEY(DRINKER, DRINK),
    CONSTRAINT LIKES_FKEY1 FOREIGN KEY(DRINK) REFERENCES ALLDRINKS(DRINK),
    CONSTRAINT LIKES_DKEY2 FOREIGN KEY(DRINKER) REFERENCES DRINKERS(DRINKER));

CREATE TABLE ORDERS(    /* Drinkers visit pubs and consumes drinks */
DRINKER     VARCHAR(30) NOT NULL,   /* Drinker name */
PUB         VARCHAR(30) NOT NULL,   /* Pub name */
ODATE       DATE        NOT NULL,   /* Order date   */
DRINK       VARCHAR(30) NOT NULL,   /* Drink name   */
DRINK_NO    DECIMAL(2)  NOT NULL,   /* A sequence number of a drink */
    CONSTRAINT ORDERS_PKEY PRIMARY KEY(DRINKER, PUB, ODATE, DRINK, DRINK_NO),
    CONSTRAINT ORDERS_FKEY1 FOREIGN KEY(PUB, DRINK) REFERENCES SERVES(PUB, DRINK),
    CONSTRAINT ORDERS_FKEY2 FOREIGN KEY(DRINKER) REFERENCES DRINKERS(DRINKER)   );

以下是 INSERT 語句的示例：

INSERT INTO LIKES VALUES('PETER', 'WHISKY', 6);
INSERT INTO LIKES VALUES('MARY', 'CHAMPAGNE', 5);

INSERT INTO ORDERS VALUES('PETER', 'LONG JOHN', STR_TO_DATE('19-APR-2020', '%d-%M-%Y'), 'BEER', 7);
INSERT INTO ORDERS VALUES('MARY', 'LONG JOHN', STR_TO_DATE('10-JAN-2020', '%d-%M-%Y'), 'RED WINE', 1);

Answer 1

如果您嘗試另一種語法，在文檔上這是為內部連接編寫的：

SELECT *
FROM A
INNER JOIN B ON A.key = B.key

可能是你想做的事情：

SELECT LIKES.DRINKER, COUNT(LIKES.RATING)
FROM LIKES 
INNER JOIN LIKES ON ORDERS.DRINK = LIKES.DRINK
GROUP BY LIKES.DRINKER; 

Answer 2

您似乎在描述LEFT JOIN

SELECT D.DRINKER, COUNT(L.RATING)
FROM DRINKERS D LEFT JOIN
     LIKES L
     ON D.DRINKER = L.DRINKER 
GROUP BY D.DRINKER; 

這就是說“保留第一個表（ DRINKERS ）中的所有行，即使是那些與ON條件不匹配的行）。