Question

SELECT p.name, p.price, COALESCE(s.size, 'Medium')
FROM product p
LEFT JOIN sizes s ON p.id = s.product
WHERE p.id = 1

如果sizes表中没有提及产品，它是否选择该产品？

  id      name      price
  1.     cap         50
  2.     glove       75


 id      product                   size
  1.     2                       'small'
  2.     2                       'large'

Answer 1

怎么样：

     SELECT p.name, p.price, IF( EXISTS(
         SELECT size
         FROM sizes s LEFT JOIN product p ON p.id = s.product
         WHERE p.id = 1
          ), s.size, 'Medium') as siz
    FROM product p
    LEFT JOIN sizes s ON p.id = s.product
    WHERE p.id = 1

Answer 2

除WHERE条件

外，您的查询没问题

SELECT p.name, p.price, COALESCE(s.size, 'Medium')
FROM product p
LEFT JOIN sizes s ON p.id = s.product

以下是SQL Fiddle的代码如果想通过查询实现其他目的，请告诉我。

加入，选择＆＃39;字符串＆＃39;如果没有找到

2 个答案: