左外連四桌

Question

這是表格結構

用戶表

Column  
fb_id   
email   
name    
fname   
lname   
gender  
fb_link 
created

推薦表

Column
id  
referred_by 
joinee  
created

當前實現的一種SQL是（示例1）

SELECT u.fb_id
    ,fb_link
    ,name
    ,r.referred_by
    ,u.created 
FROM users u 
LEFT OUTER JOIN referral r 
    ON u.fb_id=r.joinee

以上查詢結果

joinee              referer
10152250261037651   NULL
10152604594389921   NULL
10154430845000507   1518673071699780
....
....
....
....
547146735389782     NULL

這是sql（示例2）

SELECT u_joinee.fb_id joinee_fb_id
    ,u_referer.fb_id referer_fb_id
    ,u_joinee.NAME joinee_name
    ,u_referer.NAME referer_name
FROM users u_joinee
    ,users u_referer
    ,referral r_j
    ,referral r_r
WHERE u_referer.fb_id = r_r.referred_by
    AND u_joinee.fb_id = r_j.joinee

以上查詢結果

joinee_fb_id        referer_fb_id       joinee_name referer_name
10154430845000507   1518673071699780    Saselsdein  Bsdasd

我在找出來是我需要兩個名稱joinee_name和referer_name輸出。 但是我無法像我在例1中那樣在LEFT OUTER JOIN子句中使用四個表，我想同時獲取包括非引用和引用聯接的記錄，是否有任何方法可以期望所需的結果，如下所示

預期結果

joinee_fb_id        referer_fb_id       joinee_name referer_name
10154430845000507   1518673071699780    Saselsdein  Bsdasd
10154430845000347   Null                asd         NULL
10154430845000567   Null                asd asdm    NULL
10154230845000567   Null                Dsd asdm    NULL
10154330845000567   Null                sdm         NULL
101544553045000567  Null                Esd aedm    NULL

Answer 1

您可以通過為users表賦予不同的別名來兩次與users表聯接，就像對隱式內部聯接一樣。

我認為您不需要兩次加入referral 。 referral表中的單行鏈接了一個推薦人和一個參與者。

SELECT u_joinee.fb_id joinee_fb_id ,u_referer.fb_id referer_fb_id, u_joinee.name joinee_name, u_referer.name referer_name
FROM users AS u_joinee
LEFT JOIN referral AS r ON r.joinee = u_joinee.fb_id
LEFT JOIN users AS u_referer ON r.referred_by = u_referer.fb_id