如何按某些字段分组并在oracle中显示另一个字段的第一个值？(How to group by some fields and display the first value of another field in oracle?)

 分解：你想要的第一件事是产品名称： 
 
SELECT Product_Name FROM Products
 
 我们如何获得该类别？ 加入吧！ 
 
SELECT Products.Product_Name, Categories.Category_Name 
FROM Products JOIN Categories ON Products.Category_Id = Categories.Category_Id
 
 越来越接近！ 是时候引入订单历史了： 
 
SELECT Products.Product_Name, Categories.Category_Name, Sells.Sells_Date 
FROM Products 
JOIN Categories ON Products.Category_Id = Categories.Category_Id
JOIN Sells ON Sells.Sells_Id = Products.Product_Id
 
 现在我们可以引入名称，并过滤到John： 
 
SELECT Products.Product_Name, Categories.Category_Name, Sells.Sells_Date 
FROM Products 
JOIN Categories ON Products.Category_Id = Categories.Category_Id
JOIN Sells ON Sells.Sells_Id = Products.Product_Id
JOIN Customers ON Sells.Customer_Id = Customers.Customer_Id
WHERE Customers.Customer_Name = 'John'
 
 只要在脑海中一步一步。 随时随地构建每个部分的查询。 
 
 既然你提到了...... 
 
 LEFT JOIN是您处理缺失数据的方式。 它返回“left”表中的行，如果ON子句不匹配，则将NULL放在“right”表的字段中。 
 
 例如，如果John购买了没有类别的Product（ Product.Category_Id为NULL ），则使用JOIN将不会返回该购买记录，因为没有与Category_Id Null匹配的Category。 在这种情况下，您可以LEFT JOIN Categories ，在结果集中， Category_Name也将为NULL 。 

Break it down: THe first thing you want is a Product Name:
 
SELECT Product_Name FROM Products
 
How do we get the category? Join it in!
 
SELECT Products.Product_Name, Categories.Category_Name 
FROM Products JOIN Categories ON Products.Category_Id = Categories.Category_Id
 
Getting close! Time to bring in the order history:
 
SELECT Products.Product_Name, Categories.Category_Name, Sells.Sells_Date 
FROM Products 
JOIN Categories ON Products.Category_Id = Categories.Category_Id
JOIN Sells ON Sells.Sells_Id = Products.Product_Id
 
Now we can bring in the name, and filter to John:
 
SELECT Products.Product_Name, Categories.Category_Name, Sells.Sells_Date 
FROM Products 
JOIN Categories ON Products.Category_Id = Categories.Category_Id
JOIN Sells ON Sells.Sells_Id = Products.Product_Id
JOIN Customers ON Sells.Customer_Id = Customers.Customer_Id
WHERE Customers.Customer_Name = 'John'
 
Just take it one step at a time in your mind. Build the query from each part as you go.
 
Since you mentioned it...
 
LEFT JOIN is how you deal with missing data. It returns rows from the "left" table, and puts NULL in the fields of the "right" table if there is no match for the ON clause. 
 
For example, if John bought a Product with no category (Product.Category_Id was NULL), using a JOIN would not return that record of purchase because there was no matching Category with Category_Id Null. In that case, you could LEFT JOIN Categories and in the result set, Category_Name would be NULL as well.