如何加入兩個在條件對同一領域

Question

我有一個TABLE_A

| id | status1 | status2 | 
+------+---------+---------+ 
| 1 |  2 | 3 | 
+------+---------+---------+ 
| 2 |  1 | 3 | 
+------+---------+---------+ 

和表-B

| id | name | 
+------+---------+ 
| 1 | yep | 
+------+---------+ 
| 2 | nope | 
+------+---------+ 
| 3 | maybe | 
+------+---------+ 

我怎樣才能輸出到這個樣子？

1 = nope，也許;
2 =是的，也許

我想是這樣的：

SELECT * FROM table_A a 
LEFT JOIN table_B b 
ON a.status1= b.id AND a.status2= b.id" 

Answer 1

你想要做兩連接，一個爲每個狀態字段：

SELECT a.id, b1.name as name1, b2.name 
FROM table_A a LEFT JOIN 
    table_B b1 
    ON a.status1 = b1.id LEFT JOIN 
    table_B b2 
    on a.status2= b2.id; 

編輯：

用於娛樂的緣故，你可以用一個連接和聚合做到這一點：

select a.id, 
     max(case when a.status1 = b.id then b.name end) as name1, 
     max(case when a.status2 = b.id then b.name end) as name2 
from table_A a left join 
    table_B b 
    on b.id in (a.status1, a.status2) 
group by a.id; 

然而，這兩個聯接版本實際上是簡單。

Answer 2

你也可以做這樣的：

SELECT * FROM table_A a 
LEFT JOIN table_B b ON a.status1= b.id 
LEFT JOIN table_B bTmp ON a.status2= bTmp.id 

Answer 3

如果您想要一列完全爲yep, maybe和nope, maybe，則需要GROUP_CONCAT function。

你可以逃脫一個連接，如果你的「非轉動」你table_a，這就是我的回答子查詢的作用：

SELECT 
    a.id, 
    GROUP_CONCAT(b.name ORDER BY b.name DESC) 
FROM (
    SELECT id, status1 AS stat_id FROM table_a 
    UNION SELECT id, status2 FROM table_a 
) a 
INNER JOIN table_b b ON a.stat_id = b.id 
GROUP BY a.id; 

的ORDER BY在GROUP_CONCAT確保yep來maybe之前nope在逗號分隔列表中的maybe之前。

Answer 4

select a.id, b1.name, b2.name from tableB as b1 
inner join TableA as a 
on b1.id = a.status1 
inner join TableB as b2 
on b2.id = a.status2