SQL Server - 多个 FROM 关键字？

Question

搜索词对于谷歌显然是模棱两可的。我正在查看一个 SQL 调用，它有 2 个 FROM 关键字？我以前从未见过这个，有人可以解释一下吗？

SELECT TOP(5)  SUM(column) AS column, column
FROM ( SELECT DISTINCT column, column, column
FROM ((((((table table 
INNER JOIN table table ON (column = column
AND column = 2 
AND column != '' )) 
INNER JOIN table table ON (column   = column 
AND (column = 144 OR column = 159 OR column = 162 OR column = 164 OR column = 163 OR column  = 1 OR column = 2 OR column = 122 OR column = 155 OR column = 156 )))
inner join table table ON (column = column 
AND column  = 0 ))
INNER JOIN table  ON (column = column  ))
INNER JOIN table table ON ( column  = column  
AND (column   = 102 OR column  = 103 )))
INNER JOIN table table ON (column  = column  ))) TempTable  
GROUP BY column  ORDER BY column  desc 

您会注意到多个 FROM 关键字。它运行得很好。只是好奇目的是什么。

Answer 1

这称为子查询。您可以在主查询中使用 subquery 所以子查询做了多个FORM子句。