MySQL：有没有办法有条件地LEFT JOIN？ （或类似？）

Question

我正在编写查询以获取products表中的所有产品，并且每个产品的销售价格 IF 存在一条记录specials表格中的该项目。

我正在寻找的是：

SELECT * FROM products P
IF (S.specials_date_available <= NOW() AND S.expires_date > NOW())
{ // The sale has started, but has not yet expired
    LEFT JOIN specials S
      ON P.products_id = S.products_id
}

我意识到MySQL不是一种编程语言，但有没有办法创建一个导致上述逻辑等效的查询？

结果集应如下所示：

 ID    Name         Price     Sale Price
 1     Widget A     10.00     (empty, because this item has no sale record)
 2     Widget B     20.00     15.45 (this item is currently on sale)
 3     Widget C     22.00     (empty - this item was on sale but the sale expired)

Answer 1

是的，您可以将条件移至查询的JOIN ON部分。

SELECT *
FROM products P
LEFT JOIN specials S
     ON P.products_id = S.products_id AND
        S.specials_date_available <= NOW() AND
        S.expires_date > NOW()

Answer 2

SELECT * FROM products P
  LEFT JOIN specials S
    ON P.products_id = S.products_id AND S.specials_date_available <= NOW() AND S.expires_date > NOW()