使用 SQL CASE/WHEN 输出进行计算

Question

我想使用以下查询将各种货币的金额转换为美元：

SELECT 
    currency, 
    CASE currency 
      WHEN 'SAR' THEN 0.266
      WHEN 'EGP' THEN 0.113
      WHEN 'USD' THEN 1
      WHEN 'JOD' THEN 1.411
      WHEN 'GBP' THEN 1.311
      WHEN 'BHD' THEN 2.652
      WHEN 'AED' THEN 0.272
      WHEN 'EUR' THEN 1.111
      WHEN 'QAR' THEN 0.275
      WHEN 'KWD' THEN 3.315
      ELSE 0 
    END as in_usd,
    SUM(amount)*in_usd as total_in_usd
FROM 
    mytable
WHERE 
    created_at >= current_date - '7 days'::INTERVAL 
GROUP BY 
    currency

这会失败并显示消息in_usd does not exist。

预期输出是货币列表及其总数：

currency total_in_usd
USD      100.00
AED      59.00
GBP      143.01
...

Answer 1

将CASE 表达式部分放入派生表中。对结果执行GROUP BY：

select currency, in_usd, SUM(amount) * in_usd as total_in_usd
from
(
SELECT 
    currency, 
    CASE currency 
      WHEN 'USD' THEN 1
      WHEN 'SAR' THEN 0.266
      WHEN 'EGP' THEN 0.113
      WHEN 'USD' THEN 1
      WHEN 'JOD' THEN 1.411
      WHEN 'GBP' THEN 1.311
      WHEN 'BHD' THEN 2.652
      WHEN 'AED' THEN 0.272
      WHEN 'EUR' THEN 1.111
      WHEN 'QAR' THEN 0.275
      WHEN 'KWD' THEN 3.315
      ELSE 0 
    END as in_usd,
    amount
FROM 
    mytable
WHERE 
    created_at >= current_date - '7 days'::INTERVAL 
)
GROUP BY 
    currency, in_usd

Answer 2

您几乎拥有正确的结构 - 一个小的调整应该可以使其正常工作。求和时乘以货币换算 -

SELECT 
    currency, 
    SUM(amount)*
      (CASE currency 
          WHEN 'SAR' THEN 0.266
          WHEN 'EGP' THEN 0.113
          WHEN 'USD' THEN 1
          WHEN 'JOD' THEN 1.411
          WHEN 'GBP' THEN 1.311
          WHEN 'BHD' THEN 2.652
          WHEN 'AED' THEN 0.272
          WHEN 'EUR' THEN 1.111
          WHEN 'QAR' THEN 0.275
          WHEN 'KWD' THEN 3.315
          ELSE 0 
      END) as total_in_usd
FROM 
    mytable
WHERE 
    created_at >= current_date - '7 days'::INTERVAL 
GROUP BY 
    currency

Answer 3

来自documentation：

直到整个目标列表被解析后，别名才会被识别，这意味着您不能在目标列表中的其他地方引用别名。例如，以下语句将失败：

select (qtysold + 1) as q, sum(q) from sales group by 1;
ERROR:  column "q" does not exist

您可以使用子选择解决此问题：

SELECT inner.currency, inner.in_usd, inner.sum_amount*inner.in_usd as total_in_usd
from (
    SELECT 
        currency, 
        CASE currency 
          WHEN 'SAR' THEN 0.266
          WHEN 'EGP' THEN 0.113
          WHEN 'USD' THEN 1
          WHEN 'JOD' THEN 1.411
          WHEN 'GBP' THEN 1.311
          WHEN 'BHD' THEN 2.652
          WHEN 'AED' THEN 0.272
          WHEN 'EUR' THEN 1.111
          WHEN 'QAR' THEN 0.275
          WHEN 'KWD' THEN 3.315
          ELSE 0 
        END as in_usd,
        SUM(amount) as sum_amount
    FROM 
    mytable
WHERE 
    created_at >= current_date - '7 days'::INTERVAL 
GROUP BY 
    currency) inner