SQL 检查是否有 NULL 的重复项不 return

Question

我正在使用 2 个单独的查询来检查重复项，并且这两个查询在执行时应该给出相同的答案，但事实并非如此。下面是我的数据库 table 数据的示例：

id   Name   Age  Group SeatNo
------------------------------------------
1    Alpha  11    A    NULL
2    Bravo  12    A    1
3    Alpha  11    B    NULL

这是我的第一个查询，其中显示有重复项

SELECT count(*)
FROM Test AS ta
JOIN Test AS tb ON ta.name=tb.name AND ta.age=tb.age
WHERE ta.GroupName='A'
AND tb.GroupName='B'

这是我的第二个显示零重复的查询

SELECT count(*)
FROM Test AS ta
JOIN Test AS tb ON ta.name=tb.name AND ta.age=tb.age AND ta.SeatNo=tb.SeatNo
WHERE ta.GroupName='A'
AND tb.GroupName='B

在查看查询和 table 中的数据后，似乎 seatNo 中的 NULL 影响了第二个查询并导致它 return 0 次重复。是否有任何解决方案可以搜索重复项，即使它是 NULL？

Answer 1

对于第二个查询，您可以在 JOIN 中添加一个过滤条件

SELECT count(*)
FROM Test AS ta
JOIN Test AS tb ON ta.name=tb.name AND 
                   ta.age=tb.age AND 
                   ( ta.SeatNo=tb.SeatNo OR (ta.SeatNo IS NULL AND tb.SeatNo IS NULL)
WHERE ta.GroupName='A'
AND tb.GroupName='B

Answer 2

你已经学会了NULL <> NULL。你能做些什么？

好吧，ANSI SQL 有一个用于此目的的比较器 IS DISTINCT FROM。所以，你可以这样写：

SELECT count(*)
FROM Test AS ta JOIN
     Test AS tb
     ON ta.name = tb.name AND ta.age = tb.age AND
        NOT ta.SeatNo IS DISTINCT FROM tb.SeatNo
WHERE ta.GroupName = 'A' AND
      tb.GroupName = 'B';

但是，大多数数据库不支持。更一般的形式是：

SELECT count(*)
FROM Test AS ta JOIN
     Test AS tb
     ON ta.name = tb.name AND ta.age = tb.age AND
        (ta.SeatNo = tb.SeatNo OR (ta.SeatNo IS NULL AND tb.SeatNo IS NULL) )
WHERE ta.GroupName = 'A' AND
      tb.GroupName = 'B';

另一种方法使用 UNION ALL 和聚合。以下获取重复项：

select name, age, SeatNo
from ((select name, age, SeatNo, 'a' as which
       from test
       where GroupName = 'A'
      ) union all
      (select name, age, SeatNo, 'B' as which
       from test
       where GroupName = 'B'
      )
     ) ab
group by name, age, seatno
having count(distinct which) = 2;

然后您可以将其用作子查询来获取计数：

select count(*)
from (select name, age, SeatNo
      from ((select name, age, SeatNo, 'a' as which
             from test
             where GroupName = 'A'
            ) union all
            (select name, age, SeatNo, 'B' as which
             from test
             where GroupName = 'B'
            )
           ) ab
      group by name, age, seatno
      having count(distinct which) = 2  
     ) ab;

Answer 3

您的第二个查询应该检查 NULL。试试，

SELECT count(*)
FROM Test AS ta
JOIN Test AS tb ON ta.name=tb.name AND ta.age=tb.age AND ISNULL(ta.SeatNo,'')=ISNULL(tb.SeatNo,'')
WHERE ta.GroupName='A'
AND tb.GroupName='B

SQL 检查是否有 NULL 的重复项不 return

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three-valued-logic