Question

假设我们有这些（从更复杂的一个简化）示例表：

== st ==    == pr ===   == rn ===  <– tables
sta   pg    pg    rou   sta   rou  <– fields
========    =========   =========
H1    aa    aa    aaA   H1    aaA

H2    aa    aa    aaB   H2    aaB
H3    aa                H3    aaB

H4    aa    aa    aaC   H4    aaC
H5    aa                H5    aaC
H6    aa                H6    aaC

H7    aa

H8    bb    bb    NULL

我想用左内连接执行此（也简化）查询：

SELECT st.*, pr.*, rn.*
FROM         st
INNER JOIN ( pr
LEFT  JOIN   rn  ON pr.rou = rn.rou
                AND          rn.sta = st.sta -- ERROR here
           )     ON pr.pg =           st.pg
-- filter out bad rows
WHERE ( rn.id )                          -- a: not null
   OR ( pr.rou ='aaC' AND rn.id IS NULL) -- b: no joinable rn found: choose
                                         --    by a predefined pr.rou value
   OR ( pr.rou IS NULL )                 -- c: no need to join

得到这个结果：

== st ==    == pr ===   == rn ===
sta   pg    pg    rou   sta   rou
========    =========   =========
H1    aa    aa    aaA   H1    aaA
H2    aa    aa    aaB   H2    aaB
H3    aa    aa    aaB   H3    aaB
H4    aa    aa    aaC   H4    aaC
H5    aa    aa    aaC   H5    aaC
H6    aa    aa    aaC   H6    aaC

H7    aa    aa    aaA   NULL  NULL \  H7 has no rn, so choose
H7    aa    aa    aaB   NULL  NULL  } 1 row of these at the
H7    aa    aa    aaC   NULL  NULL /  WHERE / b condition

H8    bb    bb    NULL  NULL  NULL

但是MySQL抛出了这个错误：#1054 - Unknown column 'st.sta' in 'on clause'。

我尝试解决这个问题没有成功，直到有人发布（并删除）了两次加入rn列的好主意。感谢他/她，我能够创建一个有效的解决方案：

SELECT st.*, pr.*, rn.*, rn2.*

FROM         st
INNER JOIN   pr         ON st.pg = pr.pg
LEFT  JOIN   rn         ON st.sta = rn.sta
LEFT  JOIN   rn AS rn2  ON pr.rou = rn2.rou -- the two rn's join order is important
                       AND rn.id  = rn2.id  -- if first exists, second must match or null, first null => second null
WHERE ( rn.id = rn2.id )                    -- a: both not null
   OR ( pr.rou ='aaC' AND rn.id IS NULL)    -- b: no joinable rn found: choose by predefined pr.rou value
   OR ( pr.rou IS NULL )                    -- c: no need to join

虽然这个查询有效，但它需要以正确的顺序重复连接，非常脆弱和丑陋。

你能提供一个更清洁的解决方案，它只加入rn表一次吗？

以下是sql中用于copypasting的示例数据库，如果你想玩它：

DROP    TABLE IF     EXISTS st;
CREATE  TABLE IF NOT EXISTS st (
  id  int  AUTO_INCREMENT,
  sta varchar(9),
  pg  varchar(9),
  PRIMARY KEY ( id )
)  AUTO_INCREMENT=1;

DROP    TABLE IF     EXISTS pr;
CREATE  TABLE IF NOT EXISTS pr (
  id  int  AUTO_INCREMENT,
  pg  varchar(9),
  rou varchar(9),
  PRIMARY KEY ( id )
)  AUTO_INCREMENT=1;

DROP    TABLE IF     EXISTS rn;
CREATE  TABLE IF NOT EXISTS rn (
  id  int  AUTO_INCREMENT,
  sta varchar(9),
  rou varchar(9),
  PRIMARY KEY ( id )
)  AUTO_INCREMENT=1;


INSERT INTO st 
(sta , pg ) VALUES
('H1','aa'),
('H2','aa'),
('H3','aa'),
('H4','aa'),
('H5','aa'),
('H6','aa'),
('H7','aa'),
('H8','bb');
INSERT INTO pr
( pg , rou ) VALUES
('aa','aaA'),
('aa','aaB'),
('aa','aaC'),
('bb', NULL);
INSERT INTO rn
(sta , rou ) VALUES
('H1','aaA'),
('H2','aaB'),
('H3','aaB'),
('H4','aaC'),
('H5','aaC'),
('H6','aaC');

Answer 1

我想我终于明白了：

SELECT st.*, pr.*, rn.*
FROM st
  LEFT JOIN rn ON st.sta = rn.sta
  LEFT JOIN pr ON st.pg  = pr.pg
              AND (rn.rou = pr.rou OR rn.rou IS NULL)

Answer 2

select st.*, pr.*, rn.*  
from st
left join  pr on pr.pg = st.pg
left  join   rn  on rn.sta = st.sta

Answer 3

我快速测试了它，它似乎工作。我重命名了这些列，以显示它们应该代表的内容以符合您的预期结果。

SELECT 
      st.sta,
      st.pg,
      MIN( COALESCE( prByRN.pg, pr.pg )) prpg,
      MIN( COALESCE( prByRN.rou, pr.rou )) prrou,
      MIN( rn.sta ) rnsta,
      MIN( rn.rou ) rnrou
   from
      st
         left join rn
            ON st.sta = rn.sta 
            LEFT JOIN pr prByrn
               ON rn.rou = prByrn.rou
         LEFT JOIN pr
            ON st.pg = pr.pg
   GROUP BY 
      st.sta,
      st.pg

Answer 4

此查询100％正确...

检查.....

Select st.sta,st.pg,pr.pg,pr.rou,rn.sta,rn.rou from st
    Left join pr 
                     On pr.pg=st.pg
    Left join rn 
                     On rn.sta=st.sta

您的结果是给出了这些查询...

如果有任何疑问，请尽快回复。

Answer 5

SELECT st.*, pr.*, rn.*
FROM 
    st
  LEFT  JOIN 
      ( pr                          --- the parenthesis are
      LEFT  JOIN 
        rn 
          ON  pr.rou = rn.rou
      )                             --- there for clarity
    ON  rn.sta = st.sta 
    AND pr.pg =  st.pg

MySQL使用外部引用嵌套左连接

5 个答案: