选择不同的值并按条件排除行

Question

有一个表格，其中包含

category not null

、

sub_category default null

列。

• category

  始终设置

• sub_category

  是可选的

我需要从表格中选择

distinct category,sub_category

并且

• 排除 sub_category 不为 null 和 null 的记录
• 如果只有一种

  category

  +

  <null>

的组合，请记录

category    sub_category  
girl_toys        fluffy
girl_toys        dolls
drawing          <null> -- keep it since only one null sub_category
paining          red_color
paining          <null> -- exclude since there is non null sub_category

预期结果：

category    sub_category  
girl_toys        fluffy
girl_toys        dolls
drawing          <null> -- keep is since drawing has only one null
paining          red_color -- <null> record excluded since there is non null sub_category exists 

Answer 1

多种方法中的一种：

SELECT DISTINCT category, sub_category 
FROM   tbl t1
WHERE  sub_category IS NOT NULL
   OR (sub_category IS NULL
       AND NOT EXISTS (
         SELECT FROM tbl t2
         WHERE  t2.category = t1.category
         AND    t2.sub_category IS NOT NULL
         )
      );

我假设如果有多个相同的实例，您也想保留一份副本 (

category

+

null

)。