MYSQL：如何使它在選擇查詢虛擬列？

Question

下面是我的問題，任何一個有想法如何解決這個問題：

實例查詢：

SELECT id, table2.id as global_id, table3.id as global_id 
FROM table1 
LEFT JOIN table2 
    ON (table1.id = table2.tab1_id) 
LEFT JOIN table3 
    ON (table1.id = table3.tab1_id) 
WHERE etc etc 

基本上我不想在select語句寫兩次「global_id」列，也在某些行中有id = NULL

您能否在這裏寫下工作查詢。我如何獲得global_id列中所有行的id值？

感謝

Answer 1

使用COALESCE：

SELECT table1.id, COALESCE(table2.id, table3.id) as global_id 
FROM table1 
LEFT JOIN table2 
    ON (table1.id = table2.tab1_id) 
LEFT JOIN table3 
    ON (table1.id = table3.tab1_id) 
WHERE etc etc 

Answer 2

猜你需要這樣的：

SELECT id, table2.id as global_id 
FROM table1 
LEFT JOIN table2 
    ON (table1.id = table2.tab1_id) 
WHERE etc etc 
UNION ALL 
SELECT id, table3.id as global_id 
FROM table1 
LEFT JOIN table3 
    ON (table1.id = table3.tab1_id) 
WHERE etc etc