Return 布尔值（如果存在重复项）

Question

如何 return True 如果 table 有任何重复记录，False 如果它有none。

I don't need a count, I don't need to know which rows, all I want to know is if this table has duplicates or not. That's it.

Answer 1

如果您不想列出列，那么这是一种方法：

select (count(*) <> num_distinct) as has_duplicates
from t cross join
     (select count(*) as num_distinct
      from (select distinct * from t) t
     ) tt;

如果您有主键，则更有效的方法是：

select (count(*) <> count(distinct pk)) as has_duplicates
from t;

一个相对有效的主键方法是：

select (count(*) = 1) as has_duplicates
from (select t.*
      from t
      where exists (select 1 from t t2 where t2.pk = t.pk and t2.? <> t.?)
      fetch first 1 row only
     ) t;

? 是您关心的 column/columns 重复项。

Answer 2

你可以试试

SELECT COUNT(*) > 1
FROM T
GROUP BY C1,C2...

Return 布尔值（如果存在重复项）

Return boolean if duplicates exist

sql

boolean

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