Question

想象一下，我有一个包含{id，username，firstname，lastname}

的客户的数据库表

如果我想找到有多少个不同名字的实例我可以做：

select firstname, count(*) from Customers group by 2 order by 1;

   firstname | count(*)
   ====================
   bob       |   1
   jeff      |   2
   adam      |   5

如何计算多次出现的名字数量？在伪sql中，它将类似于：

select
    COUNT(
        firstname,
        count(*) as num_occurrences
    )
from 
    Customers 
group by 2 
having num_occurrences > 1;

Answer 1

你有正确的想法：

SELECT COUNT(*)
FROM (
  SELECT firstname
  FROM Customers
  GROUP BY firstname
  HAVING COUNT(*) >= 2
)

子查询计算多次出现的名字。然后你算上所有这些名字。 HAVING子句允许您按聚合进行过滤。它就像一个WHERE子句，除了你可以使用聚合函数。

Answer 2

不需要子查询。

尝试：

SELECT firstname, COUNT(*)
  FROM Customers
 GROUP BY firstname
HAVING COUNT(*) > 1
 ORDER BY firstname

或者，按照最具代表性的名称排序：

SELECT firstname, COUNT(*) AS custcount
  FROM Customers
 GROUP BY firstname
HAVING COUNT(*) > 1
 ORDER BY custcount DESC;

Answer 3

这样就可以了：

select count(username)
  from (select username
          from Customers
         group by username
        having count(*) > 1);

SQL - 计算WHERE AGGREGATE＆gt; 1

3 个答案: