在兩個或多個表上左加入

Question

返回帶有默認值或自定義用戶值的產品列表的最佳方法是什麼？例如：

產品

product_id name    category 
--------------------------------------- 
PROD1  SM-W620NZKBDBT 2 
SOFT1  MS-UIOA2189090 2 
SOFT2  LE-UIOSAI91000 2 

設置（默認設置，可以由任何用戶使用 - user_id是nulll）

product_id title   color user_id 
---------------------------------------------- 
PROD1  Samsung   black NULL 
SOFT1  MS Office-365 NULL  NULL 

設置（用戶特定的設置，建立在默認設置頂）

product_id title   color user_id 
---------------------------------------------- 
PROD1  Samsung/WIFI dark  123 
SOFT3  MS Windows 10 N/A  123 

*如果可能的話，預期用戶設置在同一個表中

預期的產品列表的輸出與USER_ID = 123和類別= 2

product_id title   color  
-------------------------------------- 
PROD1  Samsung/WIFI dark  <- product with user settings 
SOFT1  MS Office-365 NULL  <- product with default settings 
SOFT2  LE-UIOSAI91000 NULL  <- product without settings 

產品列表和設置用戶將有非常少的項目和有限的使用。

Answer 1

SELECT TOP (1) WITH TIES 
    * 
FROM products  P 
LEFT JOIN settings S ON S.product_id = P.product_id AND (S.user_id = @user_id OR S.user_id IS NULL) 
ORDER BY ROW_NUMBER() OVER(PARTITION BY P.product_id ORDER BY S.user_id DESC) 

Answer 2

CREATE TABLE #Table1 
    ([product_id] varchar(5), [name] varchar(14), [category] int) 
; 

INSERT INTO #Table1 
    ([product_id], [name], [category]) 
VALUES 
    ('PROD1', 'SM-W620NZKBDBT', 2), 
    ('SOFT1', 'MS-UIOA2189090', 2), 
    ('SOFT2', 'LE-UIOSAI91000', 2) 
; 
CREATE TABLE #Table2 
    ([product_id] varchar(5), [title] varchar(13), [color] varchar(5), [user_id] varchar(4)) 
; 

INSERT INTO #Table2 
    ([product_id], [title], [color], [user_id]) 
VALUES 
    ('PROD1', 'Samsung', 'black', NULL), 
    ('SOFT1', 'MS Office-365', NULL, NULL) 

CREATE TABLE #Table3 
    ([product_id] varchar(5), [title] varchar(13), [color] varchar(4), [user_id] int) 
; 

INSERT INTO #Table3 
    ([product_id], [title], [color], [user_id]) 
VALUES 
    ('PROD1', 'Samsung/WIFI', 'dark', 123), 
    ('SOFT3', 'MS Windows 10', 'N/A', 123) 

SELECT 
A.PRODUCT_ID,ISNULL(B.TITLE,A.NAME) TITLE,B.COLOR 
FROM #TABLE1 A LEFT JOIN 
#TABLE3 B ON A.PRODUCT_ID=B.PRODUCT_ID 
AND USER_ID = 123 
ORDER BY user_id DESC 

Answer 3

試試這個：

Declare @Products Table (product_id varchar(25) , name varchar(50) , category int); 
Declare @Settings Table (product_id varchar(25) , title varchar(50) , color varchar(25) , user_id int); 

Insert into @Products values 
('PROD1' , 'SM-W620NZKBDBT' , 2) , 
('SOFT1' , 'MS-UIOA2189090' , 2) , 
('SOFT2' , 'LE-UIOSAI91000' , 2) ; 

Insert into @Settings values 
('PROD1' , 'Samsung' , 'black' , Null), 
('SOFT1' , 'MS Office-365' , Null , Null), 
('PROD1' , 'Samsung/WIFI' , 'Dark' , 123), 
('SOFT3' , 'MS Windows 10' , 'N/A' , 123); 


SELECT A.product_id product_id , 
     coalesce(A.Name,B.title) Tile, 
     B.Color 
From @Products A Left join @Settings B ON A.product_id = B.product_id and USER_ID = 123; 

Demo。