在这个SQL语句中使用双内连接的原因是什么？

Question

我正在查看这个旧的 SQL 查询。我无法得到的一点是为什么它在同一列上两次内部连接同一个表。我说的是 Table1 和 Table1 以别名“Table1Alias”加入，

SELECT DISTINCT othercolumns,
                Table1Alias.columna
FROM   maintable
       INNER JOIN secondarytable
               ON maintable.id1 = secondarytable.a_id1
       INNER JOIN table1
               ON secondarytable.id2 = table1.id3
       INNER JOIN table1 Table1Alias
               ON secondarytable.id2 = Table1Alias.id3
       INNER JOIN thirdtable
               ON table1.id4 = thirdtable.id5
       INNER JOIN fourthtable
               ON thirdtable.id6 = fourthtable.id7
       INNER JOIN fivetable
               ON thirdtable.id8 = fivetable.id9
       INNER JOIN sixthtable
               ON Table1Alias.columna = sixthtable.id10
       LEFT JOIN seventhtable
              ON thirdtable.id11 = seventhtable.id12
WHERE  LEFT(secondarytable.type123, 2) BETWEEN '01' AND '09'
       AND secondarytable.type456 = 'cate'
       AND table1.type = '0'
       AND Table1Alias.columna = 'conn'

Answer 1

像这样重写查询可能会有所帮助，因此很明显 2 个连接是不同的，即连接是到不同的子集（同一表的）：

FROM   maintable 
       INNER JOIN secondarytable 
               ON maintable.id1 = secondarytable.a_id1 
       INNER JOIN table1 
               ON secondarytable.id2 = table1.id3 
              AND table1.type = '0' 
       INNER JOIN table1 Table1Alias 
               ON secondarytable.id2 = Table1Alias.id3 
              AND Table1Alias.columna = 'conn' 
       INNER JOIN
       ...
WHERE  LEFT(secondarytable.type123, 2) BETWEEN '01' AND '09' 
       AND secondarytable.type456 = 'cate' 

Answer 2

查看该where子句，被指向的行table1要求列type= '0'，而被指向的行table1alias要求列columna= 'conn'。

也许table1同一行有多行id3？