使用唯一字段值对记录进行计数

Question

源表

假设我有一个名为MyTable的表，其内容为：

+----------+------+
| Category | Code |
+----------+------+
|    A     | A123 |
|    A     | B123 |
|    A     | C123 |
|    B     | A123 |
|    B     | B123 |
|    B     | D123 |
|    C     | A123 |
|    C     | E123 |
|    C     | F123 |
+----------+------+

我正在尝试计算每个类别唯一的Code值的数量。

---

所需结果

对于上面的示例，结果将是：

+----------+-------------+
| Category | UniqueCodes |
+----------+-------------+
|    A     |           1 |
|    B     |           1 |
|    C     |           2 |
+----------+-------------+

由于C123对于A是唯一的，D123对于B是唯一的，并且E123和F123对于{{1 }}。

---

我尝试过的东西

我可以使用以下查询获得单个类别（例如C）的结果：

C

但是，虽然我可以对每个类别的查询进行硬编码，但似乎无法构造一个查询来为每个可能的SELECT COUNT(a.Code) AS UniqueCodes FROM ( SELECT MyTable.Code FROM MyTable WHERE MyTable.Category = "C" ) a LEFT JOIN ( SELECT MyTable.Code FROM MyTable WHERE MyTable.Category <> "C" ) b ON a.Code = b.Code WHERE b.Code IS NULL 值进行计算。

这是我尝试过的：

Category

但是，SELECT c.Category, ( SELECT COUNT(a.Code) FROM ( SELECT MyTable.Code FROM MyTable WHERE MyTable.Category = c.Category ) a LEFT JOIN ( SELECT MyTable.Code FROM MyTable WHERE MyTable.Category <> c.Category ) b ON a.Code = b.Code WHERE b.Code IS NULL ) AS UniqueCodes FROM ( SELECT MyTable.Category FROM MyTable GROUP BY MyTable.Category ) c 不在嵌套c.Category查询的范围内定义。

有人可以建议我如何获得期望的结果吗？

Answer 1

我将使用NOT EXISTS并进行聚合：

select category, count(*)
from MyTable t
where not exists (select 1 from MyTable t1 where t1.code = t.code and t1.category <> t.category)
group by category;

Answer 2

您可以使用两种聚合级别：

select minc as category, count(*)
from (select code, min(category) as minc, max(category) as maxc
      from t
      group by code
     ) as c
where minc = maxc
group by minc;

Answer 3

这也可以：

select category, count(*) from(
    select a.category,  b.count from mytable a join (
        select code, count(category) as count 
        from mytable 
        group by code 
        having count(category) = 1
    ) b on b.code = a.code
) c group by category

Answer 4

从@isaace的答案中吸取教训，我也想到了-

SELECT MyTable.Category, COUNT(*)
FROM 
    MyTable INNER JOIN 
    (SELECT Code FROM MyTable GROUP BY Code HAVING COUNT(Category) = 1) a 
    ON MyTable.Code = a.Code
GROUP BY MyTable.Category