使用复合主键查询 table，排除特定行

Question

product_id | category
---------------------
1          | A
1          | B
2          | A
3          | B

我想知道哪些唯一 ID 属于类别 A 但不属于类别 B。在上面的示例中，只有 id: 2 是有效结果

我想出了这个，但没有按预期工作：

SELECT id
FROM table
WHERE category = 'A' AND category <> 'B'
GROUP BY id

一个有效的方法是使用子查询

SELECT id
FROM table
WHERE category = 'A' 
  AND id NOT IN (SELECT id
                 FROM table
                 WHERE category = 'B')

是否可以通过单个查询完成此操作？

Answer 1

使用 self-join:

SELECT t1.id
FROM table AS t1
LEFT JOIN table AS t2 ON t1.id = t2.id AND t2.category = 'B'
WHERE t1.category = 'A' and t2.id IS NULL

另一种方法是分组：

SELECT id
FROM table
WHERE category IN ('A', 'B')
GROUP BY id
HAVING MAX(category) = 'A'

Answer 2

在 HAVING 子句中使用聚合和条件：

SELECT id
FROM tablename
GROUP BY id
HAVING SUM(category = 'A') > 0
   AND SUM(category = 'B') = 0;

或：

SELECT id
FROM tablename
WHERE category IN ('A', 'B')
GROUP BY id
HAVING SUM(category = 'B') = 0;

Query table with composite primary key, excluding specific rows