[英]join multiple times in SQL
这些是2个表。 Tech_data:
Id Tech Agent1_id Agent2_ID
1 JAVA 1 2
2 SQL 3 4
Agent_table
Id Name
1 Mike
2 John
3 Jim
4 Baron
我需要编写一个查询以带来以下输出
TECH_ID Tech Agent1_Name Agent2_Name
1 Java Mike John
2 SQL Jim Baron
我在tech_id = agent1_id上写了LEFT OUTER JOIN ON,但是我不知道如何在ON条件下加入2个ID。
为了避免必须对同一张表进行多次联接,可以取消透视,联接然后进行透视(然后,如果您有50个ID列,则仍然只需要执行一次联接):
Oracle 11g R2架构设置 :
CREATE TABLE Tech_data (Id, Tech, Agent1_id, Agent2_ID ) AS
SELECT 1, 'JAVA', 1, 2 FROM DUAL UNION ALL
SELECT 2, 'SQL', 3, 4 FROM DUAL;
CREATE TABLE Agent_table ( Id, Name ) AS
SELECT 1, 'Mike' FROM DUAL UNION ALL
SELECT 2, 'John' FROM DUAL UNION ALL
SELECT 3, 'Jim' FROM DUAL UNION ALL
SELECT 4, 'Baron' FROM DUAL;
查询1 :
SELECT *
FROM (
SELECT t.id,
t.tech,
t.num,
a.name
FROM (
SELECT *
FROM tech_data
UNPIVOT ( Agent_ID FOR num IN ( Agent1_id AS 1, Agent2_id AS 2 ) )
) t
INNER JOIN Agent_table a
ON ( t.agent_id = a.id )
)
PIVOT ( MAX( name ) FOR num IN ( 1 AS Agent1_Name, 2 AS Agent2_Name ) )
结果 :
| ID | TECH | AGENT1_NAME | AGENT2_NAME |
|----|------|-------------|-------------|
| 1 | JAVA | Mike | John |
| 2 | SQL | Jim | Baron |
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