使用SQL CROSS JOIN

Question

我有一个包含调查数据的表，两个数据点是Ethnicity和Flavor_Pref。 Flavor_Pref保持整数1,2,3,4,5。 1 =不喜欢非常多，5喜欢非常多。

 Ethnicity            Flavor_Pref
 African American         3
 Caucasian                2
 Asian                    4
 Hispanic                 1
 African American         3
 Caucasian                4
 Asian                    5
 Hispanic                 2
 African American         4
 Caucasian                1
 Asian                    4
 Hispanic                 2
 African American         3
 Caucasian                2
 Asian                    2
 Hispanic                 1

我想运行一个查询来获得4列，每个族群一个。 每个组都有不同数量的回复。

这就是我正在做的事情：我获得了超过1M的结果......我只有400个调查。

  select  AA.Flavor_Pref as AA,H.Flavor_Pref as H,C.Flavor_Pref AS C,
  A.Flavor_Pref AS A  from
  (SELECT ETHNICITY,Flavor_Pref FROM FLAVORS WHERE ETHNICITY = 'AFRICAN AMERICAN')AS AA
  CROSS JOIN
  (SELECT ETHNICITY,Flavor_Pref FROM FLAVORS WHERE  ETHNICITY = 'HISPANIC') AS H
  CROSS JOIN 
  (SELECT ETHNICITY,Flavor_Pref FROM FLAVORS WHERE ETHNICITY = 'CAUCASIAN') AS C
  CROSS JOIN 
  (SELECT ETHNICITY,Flavor_Pref FROM FLAVORS WHERE  ETHNICITY = 'ASIAN' ) AS A

我正在寻找的是：在这种情况下，西班牙裔结果较少，因此没有任何报道。

   African Americans            Hispanic        Caucasian          Asian
         3                         1                2               4
         3                         2                4               5
         4                         2                1               5
         .                         .                .               .
         .                         .                .               .
         .                         .                .               .
         3                                          2               4
         2                                          1               1  

Answer 1

你所寻找的是PIVOT中列荷兰国际集团行。 以下是执行此操作的标准方法，它将适用于所有RDBMS：

SELECT 
  MAX(CASE WHEN Ethnicity = 'African American' THEN Flavor_Pref END)
     AS 'African Americans',
  MAX(CASE WHEN Ethnicity = 'Hispanic' THEN Flavor_Pref END)
     AS 'Hispanic',
  MAX(CASE WHEN Ethnicity = 'Caucasian' THEN Flavor_Pref END)
     AS 'Caucasian',
  MAX(CASE WHEN Ethnicity = 'Asian' THEN Flavor_Pref END)
     AS 'Asian'
FROM @flavors 
GROUP BY Flavor_Pref

这是一个现场演示

Answer 2

你想要每个种族最流行的味道。 您可以使用排名功能在大多数数据库中执行此操作：

select max(case when ethnicity = 'African American' and seqnum = 1 then flavor_pref end) as AfricanAmerican,
       max(case when ethnicity = 'Hispanic' and seqnum = 1 then flavor_pref end) as Hispanic,
       max(case when ethnicity = 'Caucasian' and seqnum = 1 then flavor_pref end) as Caucasian,
       max(case when ethnicity = 'Asian' and seqnum = 1 then flavor_pref end) as Asian
from (select t.ethnicity, t.flavor_pref, cnt,
             row_number() over (partition by t.enthnicity order by cnt desc) as seqnum
      from (select t.ethnicity, t.flavor_pref, count(*) as cnt
            from t
            group by t.ethnicity, t.flavor_pref
           ) t
     ) ts