MYSQL：如何使其在選擇查詢中成為虛擬列？

Question

以下是任何人有主意如何解決此問題的我的問題：

查詢示例：

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語句中寫兩次“ global_id”列，並且在某些行中也有id = NULL

你能在這里寫工作查詢嗎？ 如何獲取global_id列中所有行的ID值？

謝謝

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

猜猜你需要這個：

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