表格之間的外連接

Question

有一個表table1具有列hId。這也可以包含數字，也包括0和null。一些加盟關係之後，每個table1.hId有full name，但只有當hId !=0 or null

然後table2.Hpid = table1.hId -- but only if hId is not null or != 0

在此之後，table2.exId = table3.Id，最後，table3.lId = table4.Id

我已經試過

SELECT 
    (SELECT table4.fullName 
    FROM table4 
    WHERE table4.Id = table3.lId) 
FROM table1 
JOIN table2 ON table1.hId = table2.hpId 
JOIN table3 ON table3.Id = table2.exId 

但通過這種方式，我只獲得了行數table1.id = table2.hId。我相信我必須使用case聲明（並且當table.hId = 0 or null =>然後打印'錯誤'），但我怎麼能實現這一點？

Answer 1

我無法理解的目標，而是嘗試去左聯接：

SELECT ISNULL(table4.Id, 'ERROR') AS Table4Id 
     , table4.fullName 
FROM table1 
LEFT JOIN table2 ON table1.holidayparkid2 = table2.Id 
LEFT JOIN table3 ON ed.Id = table2.exId <-- what's ed.Id? 
LEFT JOIN table3 ON table4.Id = table3.lId; 

Answer 2

SELECT CASE WHEN (table.hId = 0 OR table.hId IS NULL) THEN 'ERROR' 
      ELESE table4.fullName 
     END AS fullName 
FROM table1 
JOIN table2 ON table1.holidayparkid2 = table2.Id 
JOIN table3 ON ed.Id = table2.exId 
JOIN table4 ON table4.Id = table3.lId 
--WHERE table1.hId!=0 AND table1.hId IS NOT NULL --not need with case