Question

我有这个查询并且它有效，但我知道这可以简化：

SELECT class, COUNT(class), publisher FROM books WHERE publisher = 'Company ABC' GROUP BY class
UNION
SELECT class, COUNT(class), publisher FROM books WHERE publisher = 'Company DEF' GROUP BY class
UNION
SELECT class, COUNT(class), publisher FROM books WHERE publisher = 'Company GHI' GROUP BY class
UNION
SELECT class, COUNT(class), publisher FROM books WHERE publisher = 'Company JKL' GROUP BY class
UNION
SELECT class, COUNT(class), publisher FROM books WHERE publisher = 'Company MNO' GROUP BY class;

它也是硬编码但我真的希望查询在表中寻找不同的公司：

SELECT DISTINCT publisher FROM books;

结果：

publisher
Company ABC
Company DEF
Company GHI
Company JKL
Company MNO

Answer 1

使用以下查询

SELECT class, COUNT(class), publisher 
FROM books 
GROUP BY class,publisher

Answer 2

您的查询格式错误。您的DBMS应该拒绝它并出现错误。你说它＆＃34;工作＆＃34;，所以我猜它做你想做的事，但我不确定我能告诉你如何改进它。

基本问题是

SELECT class,
       COUNT(class) as N,
       publisher
FROM books
GROUP BY class

使用聚合（COUNT）时，GROUP BY子句必须包含SELECT子句中的每个非聚合。在你的情况下，它必须说

GROUP BY class, publisher

如果您只说GROUP BY class，那么该计数代表什么？我希望DBMS有点聪明＆＃34;：在您的查询中，publisher是单个值，因此COUNT是非NULL {{1}的数量该发布者的每个类的值。

相当于

class

如果这样做你想要的，那肯定是一种改进。它是标准的SQL，几乎肯定会执行得更快。

优化/简化此SQL查询

2 个答案: