如何编写涉及多个选择的 Oracle SQL 查询

Question

我有这个查询，它提供了商品名称、平均价格和特定年份的计数，如下所示。

name      Avg_price_2019    count
---------------------------------
X         23.9              234
Y         21.8               59

SQL：

SELECT 
    AVG(Amount) Avg_price_2019, name 
FROM
    (SELECT 
         name, SUM(price_amount) Amount, COUNT(*) 
     FROM 
         myTable
     WHERE
         (To_date("Activity Date", 'mm-dd-yyyy') >= TO_DATE('09/01/2019', 'mm/dd/yyyy'))
         AND (To_date("Activity Date", 'mm-dd-yyyy') <= TO_DATE('09/17/2019','mm/dd/yyyy')) 
     GROUP BY
         name)
GROUP BY
    name;

我希望它返回更多年，如下所示

name | Avg price 2018 | count | Avg price 2019 | count

对于 2018 年的结果，我需要相同的查询，只是更改年份。

如何在同一个查询中组合这两个选择以产生上述结果？

Answer 1

您似乎想要条件聚合。您的查询很可能简化如下：

SELECT
    name 
    AVG(CASE WHEN EXTRACT(YEAR FROM Activity_Date) = 2019 THEN Amount END) avg_price_2019,
    SUM(CASE WHEN EXTRACT(YEAR FROM Activity_Date) = 2019 THEN 1 ELSE 0 END) count_2019,  
    AVG(CASE WHEN EXTRACT(YEAR FROM Activity_Date) = 2018 THEN Amount END) avg_price_2018,
    SUM(CASE WHEN EXTRACT(YEAR FROM Activity_Date) = 2018 THEN 1 ELSE 0 END) count_2018
FROM myTable 
GROUP BY name

---

您似乎将日期存储为字符串，格式为mm-dd-yyyy。您可以使用SUBSTR() 提取年份部分，例如：

SELECT
    name 
    AVG(CASE WHEN SUBSTR(Activity_Date, -4) = '2019' THEN Amount END) avg_price_2019,
    SUM(CASE WHEN SUBSTR(Activity_Date, -4) = '2019' THEN 1 ELSE 0 END) count_2019,  
    AVG(CASE WHEN SUBSTR(Activity_Date, -4) = '2018' THEN Amount END) avg_price_2018,
    SUM(CASE WHEN SUBSTR(Activity_Date, -4) = '2018' THEN 1 ELSE 0 END) count_2018
FROM myTable 
GROUP BY name

---

最后，如果要过滤一年中的特定时间段（9 月 1 日至 9 月 17 日），则可以在查询中添加 WHERE 子句。仍然假设日期存储为字符串，这可能是：

WHERE SUBSTR(Activity_Date, 1, 5) BETWEEN '09-01' AND '09-17'

Answer 2

在这种情况下，我会使用“假地图列”

select a.NAME, a.AVG. a.CNT, b.AVG, b.CNT from 
(
select '1' as MAP, NAME, AVG, CNT from ... your query [2018 table] ...
) a
left join
(
select '1' as MAP, NAME, AVG, CNT from ... your query [2019 table] ...
) b
on a.MAP = b.MAP

我没有测试代码，但我想说的是，您可以在每个表中创建一个虚拟列，然后将它们留在该列上以将它们并排放置。

为了获得更好的性能，我建议在过滤日期时间时使用“BETWEEN” 例如：DATE BETWEEN date1 和 date2

Answer 3

这是where子句：

Create Table myTable 
(

    myTableID int,
    amount int,
    price_amount int,
    to_date DateTime,
    activity_date DateTime

)

Insert Into myTable (myTableID, amount, price_amount, activity_date) values (1, 1, 2, '1/1/2019')
Insert Into myTable (myTableID, amount, price_amount, activity_date) values (1, 1, 4, '1/1/2018')

select * from myTable

Select AVG(amount) as Avg_price_2019, 
   (SELECT SUM(price_amount) FROM myTable) as test from myTable
   where activity_date >= Convert(DateTime, '09/01/2019')
   and activity_date <= Convert(DateTime, '09/17/2019')