内部联接选择结果与表

Posted 2021-05-07

如何将选择结果与另一个表连接？

示例：生成每个产品类型的聚合值的2列表

SELECT product_type, SUM(value) AS total from table1
WHERE something_something_is_true
GROUP BY product_type;

另一个名为table2的表具有产品类型的文本描述

product_type | description | more columns
---------------------------------------
1            | ....................
2            | ....................

如何将description列加入上面的选择结果，以便生成的表看起来像这样？

product_type | total | description
---------------------------------
1            | 589   | stationary
2            | 234   | closing

答案

您可以将子查询用作表表达式：

SELECT t1.product_type, total, description
FROM   (SELECT   product_type, SUM(value) AS total
        FROM     table1
        WHERE    something_something_is_true
        GROUP BY product_type) t1
JOIN   table2 t2 ON t1.product_type = t1.product_type

另一答案

将您的查询用作派生表，然后将描述表连接到它。

select t1.*, t2.description
from (
  SELECT product_type, SUM(value) AS total 
  from table1
  WHERE ...
  GROUP BY product_type
) t1 
  join table2 t2 on t1.product_type = t2.product_type

另一答案

你可以这样做：

SELECT t1.product_type, SUM(value) AS total,
       MAX(t2.description)
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t2.product_type = t1.product_type
WHERE something_something_is_true
GROUP BY t1.product_type;