Question

所以我有三个表（Products，Stock和ProductsBatches），我试图获取每个产品的第一个过期的批次（来自ProductsBatches），以及来自Products（例如产品名称）和Stock（例如总计）的一些其他信息数量）

我设法只获得每种产品的最早日期，但是当我添加其他列并加入时，它弄乱了日期，并给我各种各样的日期。

PRODUCTS  TABLE: 

   | PRODUCT ID (PK)|  NAME  | PRICE |    
   |     1          | CHEESE | 12.0  |

STOCK TABLE :

   | PRODUCT ID (PFK)| TOTAL QUANTITY |
   |     1           |      100       |

PRODUCTS BATCHES TABLE:

   | PRODUCT ID (PFK) | BATCH (PFK) | BATCH QUANTITY | BATCH EXPIRY |    
   |     1            |   XYZ       |       50       |  2019-01-01  |
   |     1            |   XZZ       |       50       |  2020-01-01  |

到目前为止，我已经知道：

| PRODUCT ID | BATCH EXPIRY |
|            |              |
|     1      | 2019-01-01   |

使用SELECT productID,min(batchExpiry) from PRODUCTSBATCHES group by (productID)

我需要什么：

|PRODUCT ID| NAME |TOTAL QUANTITY|PRICE|BATCH|BATCH QUANTITY|BATCH EXPIRY|
|     1    |CHEESE|        100   |12.0 | XYZ |  50          | 2019-01-01 |

我尝试过，但也不是很好：

SELECT DISTINCT ON (b.productID) b.productID, p.name, s.totalquantity, 
p.price, b.batch, b.batchquantity, b.batchExpiry 
FROM productsbatches as b 
INNER JOIN ( 
  SELECT productID,min(batchExpiry) from PRODUCTSBATCHES group by (productID)
) as exmin b.productID = exmin.productID 
INNER JOIN stock as s ON b.productID = s.productID 
INNER JOIN products as p ON s.productID = p.productID

谢谢。

Answer 1

您已经很亲密，但是您忘记将batchExpiry列与子查询派生的最小值batchExpiry结合在一起：

SELECT 
  p.productID, p.name, s.totalquantity, 
  p.price, b.batch, b.batchquantity, b.batchExpiry
FROM products as p
INNER JOIN stock as s ON p.productID = s.productID 
INNER JOIN productsbatches as b ON p.productID = b.productID 
INNER JOIN ( 
  SELECT productID, min(batchExpiry) as batchExpiry 
  FROM PRODUCTSBATCHES 
  GROUP BY productID
) as exmin ON b.productID = exmin.productID AND b.batchExpiry = exmin.batchExpiry

Answer 2

DISTINCT ON是正确的方法。您只需要正确使用它即可：

SELECT DISTINCT ON (p.productID) p.productID, p.name, s.totalquantity, 
       p.price, b.batch, pb.batchquantity, pb.batchExpiry
FROM products p INNER JOIN
     stock s
     ON p.productID = s.productID INNER JOIN
     productsbatches pb
     ON pb.productID = p.productID 
ORDER BY p.productID, pb.batchExpiry;

基本上，您只需要ORDER BY。子查询不是必需的。

如何从组中获取带有最小值的行，并从另一个表中获取相关的值？

2 个答案: