Question

所以我有一个衣柜桌子，上面记录着拥有彩色衬衫的人。像这样，

================================
| People   |    Shirt          |
================================
| Peter    |    red            |
| Peter    |    green          |
| Peter    |    blue           |
| Peter    |    blue           |
| John     |    red            |
| John     |    green          |
| Kenny    |    yellow         |
| Kenny    |    yellow         |
| Kenny    |    green          |
| Sam      |    yellow         |
| Sam      |    green          |
| Sam      |    purple         |
| Chris    |    red            |
| Chris    |    red            |
================================

我需要帮助查询一个人有多少衬衫，只有红色衬衫。以下查询很接近，但不是我想要的。

Select people, count(shirt) from Wardrobe where shirt='red' group by people.

结果是

===========================
| People   | count(shirt) |
===========================
| Peter    |    1         |
| John     |    1         |
| Chris    |    2         |
==========================

我想从查询中得到的结果会给我这个

===========================
| People   | count(shirt) |
===========================
| Chris    |    2         |
==========================

+++++++++++++++++++++++++++++++++++++++

更新

如果我碰巧加入多个表格以及数百万的总记录，哪种技术会更好，效率更高？

Answer 1

获取一个人的红色衬衫总数，将其与衬衫总数进行比较，因此您只返回相同的行。

SELECT People, SUM(Shirt = "red") AS redcount
FROM Wardrobe
GROUP BY People
HAVING redcount = COUNT(*)

或者只是排除任何有红色衬衫的人：

SELECT People, COUNT(*) AS redcount
FROM Wardrobe
WHERE People NOT IN (
    SELECT People
    FROM Wardrobe
    WHERE Shirt != "red")
GROUP BY People

Answer 2

更长的版本：）

select w.people, w.cnt
from (
Select people,count(shirt) as cnt
from Wardrobe 
where shirt = 'red'
group by people) w
join (
  select people, count(shirt) as d_cnt 
  from Wardrobe 
  group by people) t
on w.people = t.people  and w.cnt = t.d_cnt

Answer 3

你必须使用子查询。

SELECT People，COUNT（*）AS total FROM（SELECT DISTINCT * FROM Wardrobe WHERE shirt =＆＃39; red＆＃39; GROUP BY People）;

MYSQL分组和过滤过滤器

3 个答案: