SQL左外部联接的意外输出

您没有做错任何事。这就是JOIN的行为

它是左边的行数X右边的行数，在您的情况下为3 x 2 = 6。

6 x 200 = 1200

试试这个

Select rolls.height,rolls.type, SUM(rolls.weight) as W, rollsout.Ww FROM rolls  LEFT JOIN       (Select height,type, SUM(weight) as Ww          From rollsout GROUP BY height, type       ) as rollsoutON rolls.height = rollsout.height AND rolls.type= rollsout.type GROUP BY rolls.height,rolls.type

SQLFiddle

我知道这不适用于SQL Server，但适用于MySQL