计算char在已发生少于多次的表中出现的次数

Question

我正在尝试复制此表：

列出制造商工厂少于2个的城市的所有名称。我试图通过使用HAVING（*）并将它与INNER JOIN混合来实现这些方法，但我不确定我是否在正确的轨道上。

我试图通过查看堆栈上的其他问题以两种不同的方式编写代码：

1

library(tidyverse)

base = c(1.84,3.92,1.67,1.12,1.63,.62,.59)
e1 = c(.61,1.47,1.68,1.95,1.64,.61,.72)
e2 = c(.64,7.08,1.67,1.12,1.44,.46,.76)
e3 = c(.64,4.47,1.68,2.04,1.45,.4,1.35)
e4 = c(.78,1.61,1.62,1.09,1.46,.66,.76)
e5 = c(.78,.99,1.62,2.32,1.46,.73,.52)

df = data.frame(base, e1, e2, e3, e4, e5)

# calculate colwise differences
df %>% 
  map_df( ~ (. - base)) %>% 
  select(-base) %>% 
  # create id for each number
  mutate(id = row_number()) %>% 
  # convert to long format
  gather(key = "model", value = "diff", -id) -> df_dif

# plot the differences
ggplot(df_dif, aes(x = id, y = diff)) +
  geom_col(aes(fill = model), position = "dodge") +
  facet_grid(~ model) +
  theme_classic()

结果不准确但如果我将（ManID）更改为小于2，则不会有任何结果。

2

SELECT c.CityName as "City Name", m.CityID as "Number of Manufacturers"
FROM CITY4787 c 
INNER JOIN MANUFACTURER4787 m 
ON c.CityID = m.CityID
WHERE m.ManName IN (SELECT m.ManName FROM MANUFACTURER4787
GROUP BY m.ManID HAVING COUNT (ManID) > 2);

结果设法计算出现少于2但仍然不对的城市人。

我真的不明白如何合并这两个或如果我正确使用这些sql语句。这是ERD：ERD

Answer 1

根据您的架构，我认为这应该有效：

SELECT c.CityName as "City Name", COUNT(m.ManName) AS "Number of Manufacturers"
FROM City c
LEFT JOIN Manufacturer m
ON m.CityID = c.CityID
GROUP BY c.CityID
HAVING COUNT(m.ManName) < 2

Answer 2

如果我说错了，你可以尝试一下。

如果您没有预料到这一点，您能否提供一些示例数据我会编辑我的答案。

SELECT c.CityName as "City Name", COUNT(m.ManID) as "Number of Manufacturers"
FROM CITY4787 c 
LEFT JOIN MANUFACTURER4787 m ON c.CityID = m.CityID
GROUP BY c.CityName
HAVING COUNT(m.ManID) < 2