SQL中的联合与聚合

Question

我有3个SQL查询，我想使用Union将它们合并到一个表中，但尝试了不同的查询，但没有一个起作用，我缺少什么？

SELECT DISTINCT 
       place.type AS type, 
       COUNT(place.type)AS place,
       0 AS msd,
       0 AS county
FROM   place 
GROUP BY place.type;

SELECT DISTINCT 
       mcd.type, 
       0 AS place, 
       COUNT(mcd.type) AS msd, 
       0 AS county
FROM   mcd 
GROUP BY mcd.type;

SELECT DISTINCT 
       county.type, 
       0 AS place, 
       0 AS msd,
       COUNT(county.type) AS county
FROM   county 
GROUP BY county.type;

因此，最终的输出将是方案（类型，位置，mcd，县），其中类型包含来自3个表的类型的所有不同值，并且位置包含类型值出现在位置表中的次数与mcs和county相同。

Answer 1

您需要一个外部查询才能从三个查询的组合中获取type值。

这是一个示例，使用UNION ALL集合运算符组合三个查询的结果。 该查询是一个内联视图，由外部查询引用，该外部查询在type列上执行GROUP BY，在COUNT / 0列上进行SUM聚合。

  SELECT t.type
       , SUM(t.place)
       , SUM(t.msd)
       , SUM(t.county)
    FROM ( SELECT place.type AS type
                , COUNT(place.type) AS place
                , 0 AS msd
                , 0 AS county
             FROM place
            GROUP BY place.type
            UNION ALL
           SELECT mcd.type
                , 0 AS place
                , COUNT(mcd.type) AS msd
                , 0 AS county
             FROM mcd
            GROUP BY mcd.type
            UNION ALL
           SELECT county.type
                , 0 AS place
                , 0 AS msd
                , COUNT(county.type) AS county
             FROM county
            GROUP BY county.type
         ) t
     GROUP BY t.type

通过每个查询中的GROUP BY子句，不需要DISTINCT关键字。

Answer 2

看来将3表连接在一起会更适合您的要求。

SELECT coalese(place.type,mcd.type,conty.type) AS type, 
       COUNT(place.type)AS place,
       COUNT(mcd.type) AS msd, 
       COUNT(county.type) AS county
FROM   place 
FULL OUTER JOIN   mcd    ON place.type = mcd.type
FULL OUTER JOIN   county ON place.type = county.type 
GROUP BY coalese(place.type,mcd.type,conty.type)