left join連接2個查詢結果集的用法（MySQL可以）

一、連接兩個查詢結果

left join 是left outer join的簡寫，left join可以連接2個查詢結果集。我們通常的用法

SELECT 
	S.STUDENT_NAME STUDENT_NAME, S.STUDENT_NO STUDENT_NO, F.ADDRESS ADDRESS
FROM(
	SELECT 
		G.GRADE_NO GRADE_NO, A.ADDRESS
	FROM
		GRADE G, ADDRESS A
	WHERE
		G.GRADE_NO = A.GRADE_NO
) F LEFT JOIN STUDENT S ON F.STUDENT_NO = S.STUDENT_NO

　　

  這是我們最常見的用法，基本上這種連接都能滿足一般的查詢需求。這里還要說明
一下，LEFT JOIN是先連接，后過濾。也就是說，在WHERE條件過濾之前，已經進行了連
接。假如我們現在有這樣一種場景，我們需要連接STUDENT表中的所有2016屆的學生，
而不是連接STUDENT表中所有的記錄。那么我們就需要在連接之前查詢出來所有2016屆
的學生。這種場景，left join也能輕松搞定。如下：

SELECT 
	P.STUDENT_NAME STUDENT_NAME, P.STUDENT_NO STUDENT_NO, F.ADDRESS ADDRESS
FROM(
	SELECT 
		G.GRADE_NO GRADE_NO, A.ADDRESS
	FROM
		GRADE G, ADDRESS A
	WHERE
		G.GRADE_NO = A.GRADE_NO
) F LEFT JOIN 
	(
		SELECT 
			S.STUDENT_NO STUDENT_NO, S.STUDENT_NAME STUDENT_NAME
		FROM
			STUDENT S
		WHERE
			S.YEAR = '2016'
	)P ON P.STUDENT_NO = F.STUDENT_NO

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版權聲明：本文為CSDN博主「春秋戰國程序猿」的原創文章，遵循 CC 4.0 BY-SA 版權協議，轉載請附上原文出處鏈接及本聲明。
原文鏈接：https://blog.csdn.net/reggergdsg/article/details/66475187

二、連接多個查詢結果

項目中遇到這樣的問題，需要按指定的時間區間從10個數據庫表中查詢到關於10個司法局的業務數量，案件數量等11列信息。再對這些結果排序。 

  那么我們怎么通過一個SQL語句來解決呢？

  下面介紹下SQL方法：

select 
a.*,
a.id,
b.id, 
c.price 
from author a 
left join book b on a.id=b.id 
left join order c on a.id=c.id

　　

    使用Left join -on語句將3張表鏈接到了一起。對於項目的解決辦法，可以將book，order，auther替換成sql語句，即select出來的表，再做一次left join

    SQL語句如下

 

 1 select 
 2 a.MEDIATIONCOMMITTEE,
 3 a.tiaojieaj,
 4 b.shangbaosl,
 5 c.paichafk,
 6 d.zixunrz,
 7 e.chunjufw,
 8 f.faxuanhd
 9 from (select MEDIATIONCOMMITTEE,count(1) as tiaojieaj from MBM_CASE where DATEACCEPTED <= to_date('2018-05-10 00:00:00','yyyy-mm-dd hh24:mi:ss') group by MEDIATIONCOMMITTEE) a
10 left join (select MEDIATIONCOMMITTEE,count(1) as shangbaosl from MCS_MEDIATIONCASE where REPORTDATE between to_date('2016-01-10 00:00:00','yyyy-mm-dd hh24:mi:ss') and to_date
('2018-05-10 00:00:00','yyyy-mm-dd hh24:mi:ss') and STATUS !='ToBeReported' group by MEDIATIONCOMMITTEE) b 
11 on a.MEDIATIONCOMMITTEE=b.MEDIATIONCOMMITTEE
12 left join (select MEDIATIONCOMMITTEE,count(1) as paichafk from CDS_INVESTIGATIONFEEDBAC where DATE_ between to_date('2016-01-10 00:00:00','yyyy-mm-dd hh24:mi:ss') and to_date
('2018-05-10 00:00:00','yyyy-mm-dd hh24:mi:ss') group by MEDIATIONCOMMITTEE) c
13 on a.MEDIATIONCOMMITTEE=c.MEDIATIONCOMMITTEE
14 left join (select MEDIATIONCOMMITTEE,count(1) as zixunrz from AMS_VILLAGESERVICELOG where CREATEDATE between to_date('2016-01-10 00:00:00','yyyy-mm-dd hh24:mi:ss') and to_date
('2018-05-10 00:00:00','yyyy-mm-dd hh24:mi:ss') group by MEDIATIONCOMMITTEE ) d
15 on a.MEDIATIONCOMMITTEE=d.MEDIATIONCOMMITTEE
16 left join (select MEDIATIONCOMMITTEE,count(1) as chunjufw from WWS_CONSULT where DATE_ between to_date('2016-01-10 00:00:00','yyyy-mm-dd hh24:mi:ss') and to_date
('2018-05-10 00:00:00','yyyy-mm-dd hh24:mi:ss') group by MEDIATIONCOMMITTEE  ) e
17 on a.MEDIATIONCOMMITTEE=e.MEDIATIONCOMMITTEE
18 left join (select MEDIATIONCOMMITTEE,count(1) as faxuanhd from WWS_LEGALACTIVITY where STARTDATE between to_date('2016-01-10 00:00:00','yyyy-mm-dd hh24:mi:ss')
 and to_date('2018-05-10 00:00:00','yyyy-mm-dd hh24:mi:ss') group by MEDIATIONCOMMITTEE) f
19 on a.MEDIATIONCOMMITTEE=f.MEDIATIONCOMMITTEE

　　

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版權聲明：本文為CSDN博主「adam-liu」的原創文章，遵循 CC 4.0 BY-SA 版權協議，轉載請附上原文出處鏈接及本聲明。
原文鏈接：https://blog.csdn.net/qq_41664845/article/details/80763112