MySQL与特定的WHERE子句连接

Question

我有一个简单的数据库

通过此查询，我获得了所有用户和他们购买的产品数量：

SELECT
  c.id,
  cd.name,
  cd.surname,
  COUNT(p.name) as bought
FROM shopping s
 JOIN client c ON s.client_id = c.id
 JOIN client_details cd ON c.id = cd.client_id
 JOIN product p ON s.product_id = p.id
GROUP BY c.id;

它非常好，但我必须选择所有用户和购买的特定产品的数量。有了这个查询，我只有在用户购买了至少一种产品时才能得到它：

SELECT
  c.id,
  cd.name,
  cd.surname,
  COUNT(p.name) as bought
FROM shopping s
 JOIN client c ON s.client_id = c.id
 JOIN client_details cd ON c.id = cd.client_id
 JOIN product p ON s.product_id = p.id
WHERE p.name = 'sugar'
GROUP BY c.id;

我希望获得所有用户的列表，如果特定用户没有＆＃39;购买特定产品，输出应该是0。

Answer 1

使用条件聚合：

SELECT c.id, cd.name, cd.surname,
       MAX(p.name = 'sugar') as SugarFlag
FROM shopping s JOIN
     client c
     ON s.client_id = c.id JOIN
     client_details cd
     ON c.id = cd.client_id JOIN
     product p
     ON s.product_id = p.id
GROUP BY c.id;