當一列空然後從其他列獲取數據

Question

希望你能幫助。沒有顯示整個查詢，因爲我不認爲它是必需的，但想要嘗試並查看「street」中是否有空值，然後查看「free address」並將結果放在那裏。我猜它涉及到一個CASE表達式或CONCAT

當前結果

SELECT 

IdNum 
street 
free address 

from ID 


IdNum   street     free address 
    1   stack over flow    null 
    2    null     stack exchange 
    3   downing street    null 

需要的結果 - 注意無地址欄不需要展出，只是爲了演示的目的。

IdNum   street     free address 
    1   stack over flow     null 
    2   stack exchange    stack exchange 
    3   downing street     null 

非常感謝所有幫助

Answer 1

的COALESCE()功能做到這一點：

SELECT IdNum, 
     COALESCE(street, free_address) as street, 
     free_address 
FROM ID; 

注：ID是一個陌生的名字爲表和free address不是一個有效的列名（因的空間）。

COALESCE()是基本上所有數據庫都可以使用的ANSI標準函數。

Answer 2

使用COALESCE()功能，可以用另一種替換NULL值選擇：

SELECT IdNum, COALESCE(street, `free address`) AS street, `free address` 
FROM ID 

Answer 3

您可以使用CASE，COALESCE或（在SQL服務器）ISNULL：

SELECT IdNum, 
     CASE WHEN street IS NULL THEN free_address ELSE street END as street, 
     free_address 
FROM ID; 

SELECT IdNum, 
     COALESCE(street, free_address) as street, 
     free_address 
FROM ID; 

SELECT IdNum, 
     ISNULL(street, free_address) as street, 
     free_address 
FROM ID; 

Answer 4

根據您使用的數據庫，您可以使用ISNULL,NVL,IFNULL或COALESCE。

選擇這裏是正確的：SQL NULL Functions at w3schools

Answer 5

如果使用SQL Server

Select [IdNum], 
Case 
When [street] Is Null AND [free address] Is Not Null 
Then [free address] else [street] END As 'street', 
[free address] 
from ID