连接两个表 - Oracle SQL

Question

我有两张桌子，我想加入他们。下面是表格示例和结果表：

表A

ID	地点	R_ID	M_ID
1	PLACE_1	51	53
2	PLACE_2	52	54

B桌

S_ID	姓名
51	A
52	乙
53	C
54	D

输出表

ID	地点	R_NAME	M_NAME
1	PLACE_1	A	C
2	PLACE_2	乙	D

我试过查询：

SELECT
    id,
    place,
    name as r_name
FROM
    table_a 
    LEFT JOIN table_b ON r_id = s_id

查询结果

ID	地点	R_NAME
1	PLACE_1	A
2	PLACE_2	乙

但是我不知道如何加入下一专栏m_name.

Answer 1

两次加入同一张表，使用表别名来区分两者：

SELECT id,
       place,
       r.name as r_name,
       m.name as m_name
FROM   table_a a
       LEFT JOIN table_b r ON a.r_id = r.s_id
       LEFT JOIN table_b m ON a.m_id = m.s_id

其中，对于示例数据：

CREATE TABLE Table_A (ID, PLACE, R_ID, M_ID) AS
SELECT 1, 'PLACE_1', 51, 53 FROM DUAL UNION ALL
SELECT 2, 'PLACE_2', 52, 54 FROM DUAL;

CREATE TABLE Table_B (S_ID, NAME) AS
SELECT 51, 'A' FROM DUAL UNION ALL
SELECT 52, 'B' FROM DUAL UNION ALL
SELECT 53, 'C' FROM DUAL UNION ALL
SELECT 54, 'D' FROM DUAL;

输出：

ID	地点	R_NAME	M_NAME
1	PLACE_1	A	C
2	PLACE_2	乙	D

小提琴

Answer 2

你可以这样做：

with cte_r as ( 
  SELECT a.ID, a.PLACE, b.NAME
  FROM tableA a
  INNER JOIN tableB b on a.R_ID = b.S_ID
),
cte_m as (
  SELECT a.ID, a.PLACE, b.NAME
  FROM tableA a
  INNER JOIN tableB b on a.M_ID = b.S_ID
)
SELECT r.ID, r.PLACE, r.NAME as R_NAME, m.NAME as M_NAME
FROM cte_r r
LEFT JOIN cte_m m on r.ID = m.ID and r.PLACE = m.PLACE

结果：

ID  PLACE   R_NAME  M_NAME
1   PLACE_1 A       C
2   PLACE_2 B       D

这里演示

Answer 3

也可以使用子查询：

SELECT a.id,
   a.place,
   (SELECT name FROM table_b r WHERE a.r_id = r.s_id) AS r_name,
   (SELECT name FROM table_b m WHERE a.m_id = m.s_id) AS m_name
FROM table_a a;