Question

我有三种类型的数据源。它们具有相似的结构，但有所不同。例如，类型A包含名为grade的列，但类型C不包含。在执行sql选择时，我只想编写一个查询（因为要选择的项目很多，而且大多数都相同）。这意味着对于C型数据源，将没有等级。在这种情况下，如何跳过此特定选择。

我的查询的说明如下：

SELECT
ID,
SUM(Addable) AS addable_sum,
MAX(Date) AS max_date,
GROUP_CONCAT(Char, '_') AS Char
AVG(grade) AS grade --------------> this is what I want to skip when dealing with type C data since there is not such thing in it
FROM TypeCTable ---------------> such table does not have grade column
GROUP BY ID

Answer 1

是否需要联合选择，例如：

  SELECT
  ID,
  SUM(Addable) AS addable_sum,
  MAX(Date) AS max_date,
  GROUP_CONCAT(Char, '_') AS Char
  AVG(grade) AS grade 
  FROM TypeATable
  GROUP BY ID
  UNION ALL
  SELECT
  ID,
  SUM(Addable) AS addable_sum,
  MAX(Date) AS max_date,
  GROUP_CONCAT(Char, '_') AS Char
  NULL 
  FROM TypeCTable 
  GROUP BY ID

或使用UNION获得不同的结果

sql不存在时跳过选择项

1 个答案: