MYSQL：如何使其在选择查询中成为虚拟列？

Question

以下是任何人有主意如何解决此问题的我的问题：

查询示例：

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语句中写两次“ global_id”列，并且在某些行中也有id = NULL

你能在这里写工作查询吗？ 如何获取global_id列中所有行的ID值？

谢谢

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

猜猜你需要这个：

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