SQL语法:仅当结果超过X个时才选择 [英] SQL syntax: select only if more than X results

问题描述

我有一个带有度量的表，称为 measures .该表在 location 中有一列，而在对应的 value 中有第二列(简化了示例).

I have a table with measurements called measures. The table has one column for the location and a second colum for a corresponding value (example is simplified).

该表格如下所示(请注意 loc1 的2个条目):

The table looks like (note 2 entries for loc1):

location | value
-----------------
loc1     | value1
loc1     | value2
loc2     | value3
loc3     | value4
loc4     | value5

我现在想制定一个SQL查询(实际上我使用sqlite)，该查询仅返回表的前两行(即loc + value1和loc1 + value2)，因为此位置在该表中有多个条目.

i now want to formulate a SQL query (actually i use sqlite) which only returns the first two rows of the table (i.e. loc+value1 and loc1+value2), because this location has more than one entry in this table.

伪文本表述为:向我显示位置的行，这些行在整个表格中多次出现
密码:

the pseudotext formulation would be: show me the rows of the locations, which are present more than once in the whole table
pseudcode:

SELECT * from measures WHERE COUNT(location over the whole table) > 1

该解决方案可能真的很简单，但是我似乎并没有因此而崩溃.

the solution may be really simple, but somehow i seem not to crack the nut.

到目前为止，我所拥有的是一条SELECT语句，该语句返回具有多个条目的位置.下一步，我将需要与该查询返回的位置完全对应的所有行:

what i have so far is a SELECT statement, which returns locations which have more than one entry. as a next step i would need exactly all rows which correspond to the locations returned from this query:

SELECT location FROM measures GROUP BY location HAVING count(*) > 1

因此，下一步我尝试对同一张表进行JOIN并合并上面的查询，但是结果不正确.我这样尝试过，但这是错误的:

so as a next step i tried to do a JOIN with the same table and incorporate above query, but the results are incorrect. i tried it like this, but this is wrong:

select t1.location, t1.value
from 
     measures as t1
     join 
     measures as t2 on t1.location = t2.location 
group by
      t2.location 
having count(*) > 1

我们非常感谢您的帮助！

help is appreciated!

推荐答案

您正确使用HAVING，并考虑使用自联接...只是操作顺序略有不同...

You were right to use HAVING, and to think about using the self-join... just had the order of the operations slightly off...

select m1.location, m1.value
from measures m1
join (
  select location
  from measures
  group by location
  having count(*) > 1
) m2 on m2.location = m1.location

子选择将获得具有多个条目的所有位置...，然后再次将其连接到表以获取完整结果.

The sub-select gets all the locations that have more than one entry... and then this is joined to the table again to get the full results.

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