SQL查询的问题

Question

我有2个表和结果，如下图所示：MySQL DB

加入这两个表的最佳方法是什么，所以我们得到如上所示的结果。

SELECT * FROM (SELECT id, desc FROM table2) as T1 
LEFT JOIN (SELECT * FROM table1) as T2 ON T1.id = T2.id

我猜我的SQL无效。

Answer 1

使用带有coalesce的左连接来优先处理表2的值（如果存在），但如果不存在则返回表1的值。

select t1.id,
       coalesce(t2.desc, t1.desc) as desc,
       t1.d1, t1.d2
  from table1 t1
  left join table2 t2
    on t2.id = t1.id
 order by t1.id

Answer 2

您可以LEFT JOIN使用COALESCE：

SELECT t1.id, COALESCE(t2.desc, t1.desc) AS desc, t1.D1, t1.D2
FROM table1 as T1 
LEFT JOIN table2 as T2 ON T1.id = T2.id

Answer 3

您可以使用ifnull：

SELECT t1.id, ifnull(t2.desc, t1.desc) AS desc, t1.D1, t1.D2
FROM table1 as T1 
LEFT JOIN table2 as T2 ON T1.id = T2.id

coalesce或case .. when也是可能的。全部与left join

一起