如何在 SQL 中使用集合代数运算

Question

我需要找到喜欢啤酒或红酒的独特饮酒者。但是，查询必须使用集合代数运算来实现。

这是我目前所拥有的：

SELECT DRINKER 
FROM LIKES 
WHERE DRINK = 'BEER' OR 'RED WINE'; 

这不是使用集合代数运算。我对 SQL 真的很陌生，所以对此的任何帮助将不胜感激。

这是我从中获取数据的表：

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)); 

以及表'DRINKERS'：

CREATE TABLE DRINKERS ( /* All drinkers */
DRINKER VARCHAR(30) NOT NULL,
    CONSTRAINT DRINKERS_PKEY PRIMARY KEY (DRINKER));

插入语句：

INSERT INTO LIKES VALUES('TOM', 'BEER', 6);
INSERT INTO LIKES VALUES('JANUSZ', 'VODKA', 6);
INSERT INTO LIKES VALUES('JANUSZ', 'RUM', 5);
INSERT INTO LIKES VALUES('JANUSZ', 'BEER', 6);
INSERT INTO LIKES VALUES('JANUSZ', 'CHAMPAGNE', 6);
INSERT INTO LIKES VALUES('JANUSZ', 'RED WINE', 6);
INSERT INTO LIKES VALUES('JANUSZ', 'WHITE WINE', 5);
INSERT INTO LIKES VALUES('JANUSZ', 'PORT', 5);
INSERT INTO LIKES VALUES('PETER', 'CHAMPAGNE', 4);
INSERT INTO LIKES VALUES('PETER', 'COGNAC', 3);
INSERT INTO LIKES VALUES('PETER', 'RUM', 3);
INSERT INTO LIKES VALUES('PETER', 'WHISKY', 6);
INSERT INTO LIKES VALUES('MARY', 'CHAMPAGNE', 5);
INSERT INTO LIKES VALUES('MARY', 'VODKA', 1);
INSERT INTO LIKES VALUES('MARY', 'COGNAC', 4);
INSERT INTO LIKES VALUES('JOHN', 'CHAMPAGNE', 4);
INSERT INTO LIKES VALUES('JOHN', 'VODKA', 2);
INSERT INTO LIKES VALUES('JOHN', 'RUM', 6);
INSERT INTO LIKES VALUES('JOHN', 'WHISKY', 1);
INSERT INTO LIKES VALUES('JOHN', 'BEER', 6);
INSERT INTO LIKES VALUES('JAMES', 'CHAMPAGNE', 6);
INSERT INTO LIKES VALUES('JAMES', 'COGNAC', 5);
INSERT INTO LIKES VALUES('JAMES', 'RUM', 4);
INSERT INTO LIKES VALUES('SERGIEY', 'VODKA', 6);
INSERT INTO LIKES VALUES('SERGIEY', 'RUM', 6);
INSERT INTO LIKES VALUES('SERGIEY', 'CHAMPAGNE', 3);
INSERT INTO LIKES VALUES('CLAUDE', 'CHAMPAGNE', 6);
INSERT INTO LIKES VALUES('CLAUDE', 'WHITE WINE', 5);
INSERT INTO LIKES VALUES('CLAUDE', 'COGNAC', 4);
INSERT INTO LIKES VALUES('CLAUDE', 'WHISKY', 3);
INSERT INTO LIKES VALUES('CLAUDE', 'RED WINE', 6);

期望的输出：

DRINKER

JANUSZ
JOHN
TOM

Answer 1

UNION是MySQL支持的集合代​​数运算，有两种形式：UNION和UNION ALL。前者只返回集合中不同的值，后者返回所有值（因此对于您的样本数据，将返回两次JANUSZ，因为它们喜欢BEER 和RED WINE）。因此，一个集合代数查询来查找喜欢BEER 或RED WINE 的饮酒者将是：

SELECT DRINKER
FROM LIKES
WHERE DRINK = 'BEER'
UNION
SELECT DRINKER 
FROM LIKES 
WHERE DRINK = 'RED WINE'

样本数据的输出：

DRINKER
TOM
JANUSZ
JOHN
CLAUDE

（请注意，CLAUDE 也喜欢 RED WINE）

Demo on dbfiddle

您可以将ORDER BY 子句添加到UNION 以订购最终结果。如果没有一个，则无法保证结果的顺序。例如按字母排序：

SELECT DRINKER
FROM LIKES
WHERE DRINK = 'BEER'
UNION
SELECT DRINKER 
FROM LIKES 
WHERE DRINK = 'RED WINE'
ORDER BY DRINKER

输出：

DRINKER
CLAUDE
JANUSZ
JOHN
TOM

Demo on dbfiddle

Answer 2

在标准 SQL 中，集合代数运算是 UNION、INTERSECT 和 EXCEPT。对于您的需求，我认为 UNION 将是最佳选择。

由于您没有指定某人是否喜欢某款饮品的评分，因此对于此查询，我们假设 5 表示他们喜欢。

SELECT DISTINCT DRINKER
FROM (SELECT DISTINCT DRINKER
      FROM LIKES
      WHERE DRINK="BEER"
      AND RATING>5
      UNION
      SELECT DISTINCT DRINKER
      FROM LIKES
      WHERE DRINK="RED WINE"
      AND RATING>5);

假设“喜欢”的评分 >5，您将从您选择的 INSERT 语句中获得以下输出：

CLAUDE
JANUSZ
JOHN
TOM

您可以在此处阅读有关 Set 操作的更多信息：

https://en.wikipedia.org/wiki/Set_operations_(SQL)