MS-Access查詢到PostgreSQL視圖

Question

我正在將Microsoft Access查詢轉換為Postgresql視圖。 該查詢具有明顯的組件，我已經找到了合理的答案。 但是，我仍然堅持獲得最終結果：

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));

意識到我需要使用CASE SUM，這是到目前為止我已經完成的工作：

    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));

目標是計算Sec_ID具有以下內容的實例數：

• 具有（類型= LODE和狀態=活動）= SUM整數
• 具有（類型= LODE和Loc_Date在8/31/2017和9/1/2018之間）= SUM整數

我的主要問題是在新列中填充一個SUM整數

Answer 1

大小寫表達式與Access IIF（）函數等效，但是WHEN不是函數，因此不能通過傳遞一組參數來使用。 而是將其視為一個微小的where子句，它會評估一個或多個謂詞以確定要執行的操作，並且采取的行動由您在THEN之后指定的內容來確定

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
;

順便說一句，您不應該依賴MM / DD / YYYY作為Postgres中的日期

nb：聚合函數忽略NULL，請看以下示例：

+----------+
| 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 |
+----------+-------------+