MS-Access 查询到 PostgreSQL 视图

Question

我正在将 microsoft access 查询转换为 postgresql 视图。该查询具有明显的组件，我已经找到了合理的答案。但是，我仍然坚持得到最终结果：

SELECT All_Claim_Data.Sec_ID,
    Sum(IIf([Type]="LODE",IIf([Status]="Active",1,0),0)) AS LD_Actv, 
    Sum(IIf([Type]="LODE",IIf([Loc_Date]>#8/31/2017#,IIf([Loc_Date]<#9/1/2018#,1,0),0),0)) AS LD_stkd_17_18, 
    Sum(IIf([Type]="LODE",IIf([Loc_Date]>#8/31/2016#,IIf([Loc_Date]<#9/1/2017#,1,0),0),0)) AS LD_stkd_16_17,
    Sum(IIf([Type]="LODE",IIf([Loc_Date]<#1/1/1910#,IIf(IsNull([Clsd_Date]),1,(IIf([Clsd_Date]>#1/1/1900#,1,0))),0),0)) AS Actv_1900s, 
    Sum(IIf([Type]="LODE",IIf([Loc_Date]<#1/1/1920#,IIf(IsNull([Clsd_Date]),1,(IIf([Clsd_Date]>#1/1/1910#,1,0))),0),0)) AS Actv_1910s,
  FROM All_Claim_Data.Sec_ID,
  GROUP BY All_Claim_Data.Sec_ID,
  HAVING (((Sum(IIf([casetype_txt]="LODE",1,0)))>0));

意识到我需要使用 CASE SUM WHEN，这是我到目前为止所制定的：

    CREATE OR REPLACE VIEW hgeditor.vw_test AS
     SELECT All_Claim_Data.Sec_ID,
     SUM (CASE WHEN(Type='LODE' AND WHEN(Status='Active',1,0),0)) AS LD_Actv, 
     SUM (CASE WHEN(Type='LODE' AND WHEN(Loc_Date>'8/31/2017' AND Loc_Date<'9/1/2018',1,0),0),0)) AS LD_stkd_17_18,
     SUM (CASE WHEN(Type='LODE' AND WHEN(Loc_Date<'1/1/1910' AND (IsNull(Clsd_Date),1,(WHEN([Clsd_Date]>'1/1/1900',1,0))),0),0)) AS Actv_1900s
    FROM All_Claim_Data.Sec_ID,
    GROUP BY All_Claim_Data.Sec_ID,
    HAVING (((SUM(IIf(Type='LODE',1,0)))>0));

目标是统计 Sec_ID 具有以下内容的实例数：

• 有（类型 = LODE 和状态 = 活动）= SUM 整数
• 具有（类型 = LODE 和 Loc_Date 介于 2017 年 8 月 31 日和 2018 年 9 月 1 日之间）= SUM 整数

我的主要问题是获取一个 SUM 整数以填充到新列中

Answer 1

大小写表达式等效于 Access IIF() 函数，但 WHEN 不是函数，因此不能通过传递一组参数来使用它。将其视为一个很小的 ​​where 子句，它评估一个或多个谓词以确定要做什么，并且采取的操作由您在 THEN 之后指定的内容确定。

CREATE OR REPLACE VIEW hgeditor.vw_test AS
SELECT
    All_Claim_Data.Sec_ID
  , SUM( CASE
        WHEN TYPE = 'LODE' AND
            STATUS = 'Active' THEN 1
        ELSE 0
    END ) AS LD_Actv
  , SUM( CASE
        WHEN TYPE = 'LODE' AND
            Loc_Date > to_date('08/31/2017','mm/dd/yyyy') AND
            Loc_Date < to_date('09/1/2018','mm/dd/yyyy')  THEN 1
        ELSE 0
    END ) AS LD_stkd_17_18
  , SUM( CASE
        WHEN TYPE = 'LODE' AND
            Loc_Date < to_date('1/1/1910','mm/dd/yyyy')  AND
            [Clsd_Date] > to_date('1/1/1900','mm/dd/yyyy')  THEN 1
        ELSE 0
    END ) AS Actv_1900s
FROM All_Claim_Data.Sec_ID
GROUP BY
    All_Claim_Data.Sec_ID
HAVING COUNT( CASE
    WHEN Type = 'LODE' THEN 1
END ) > 0
;

顺便说一句，你不应该依赖 MM/DD/YYYY 作为 Postgres 中的日期

nb：聚合函数忽略NULL，举个例子：

+----------+
| id value |
+----------+
| 1  x     |
| 2  NULL  |
| 3  x     |
| 4  NULL  |
| 5  x     |
+----------+

select
       count(*)      c_all
     , count(value)  c_value
from t

+-------+----------+
| c_all |  c_value |
+-------+----------+
|     5 |        3 |
+-------+----------+


select
       sum(case when value IS NOT NULL then 1 else 0 end) sum_case
     , count(case when value IS NOT NULL then 1 end)      count_case
from t

+----------+-------------+
| sum_case |  count_case |
+----------+-------------+
|        3 |           3 |
+----------+-------------+