Question

此问题是此post的扩展我想用SQLite加入四个不同的表，它们只有两个共同的列。但是，假设有30多列，即不仅仅列 a - h 。请看下面的例子

表1：

a   b   lon lat
---------------
22  33  11  22

表2：

c   d   lon lat
---------------
1   2   44  45

表3

e       f       lon lat
-----------------------
NULL    NULL    100 101

表4

g       h       lon lat
-----------------------
NULL    NULL    200 201

目前的解决方案如下

SELECT a,b,NULL AS c, NULL AS d,NULL AS e, NULL AS f, NULL AS g, NULL AS h,
lon,lat
FROM table1
UNION ALL
SELECT NULL, NULL,c,d,NULL AS e, NULL AS f, NULL AS g, NULL AS h, lon,lat
FROM table2
UNION ALL
SELECT NULL, NULL,NULL,NULL,e,f, NULL AS g, NULL AS h, lon,lat
FROM table3
UNION ALL
SELECT NULL, NULL,NULL,NULL,NULL,NULL,g,h, lon,lat
from table4

结果：

+------+------+------+------+------+------+------+------+-----+-----+
|  a   |  b   |  c   |  d   |  e   |  f   |  g   |  h   | lon | lat |
+------+------+------+------+------+------+------+------+-----+-----+
| 22   | 33   | NULL | NULL | NULL | NULL | NULL | NULL |  11 |  22 |
| NULL | NULL | 1    | 2    | NULL | NULL | NULL | NULL |  44 |  45 |
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 100 | 101 |
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 200 | 201 |
+------+------+------+------+------+------+------+------+-----+-----+

DEMO

问题：如果我不只是a列直到h，而是直到z，即table1，table2，table3和table4中的许多列 - ＆gt;这将是非常耗时的，如果我必须在我的sql语句中将[NULL]作为[letter]写入任何地方，结构将不会很清楚
@zarruq是一个巨大的帮助，他建议在那种情况下，我可以使用`UNION ALL`然后`PIVOT`将列转换为行
但是，我不知道该怎么做。并且，我不知道100％他的意思。
编辑：SQLite不支持pivot：还有其他建议吗？

Answer 1

当你有30多列时，我不相信这是一种特别巧妙的方法。以下是我能做的最好的事情，使用嵌套的CTE来实现full outer joins，然后使用coalesce来选择第一个非空的lat / lon。

仍需要在最高SELECT语句中枚举所有30多个字段，但至少需要NULL AS ...的大量列表：

SELECT 
  a, b, c, d, e, f, g, h,
  coalesce(lat1, lat2, lat3, lat4) AS lat,
  coalesce(lon1, lon2, lon3, lon4) AS lon
  FROM (
    WITH t1_x_t2 AS (
        SELECT t1.*, t2.*, 
        t1.lat AS lat1, t2.lat AS lat2, t1.lon AS lon1, t2.lon AS lon2
        FROM table1 t1 LEFT OUTER JOIN table2 t2 ON 0
      UNION ALL
        SELECT t1.*, t2.*, 
        t1.lat AS lat1, t2.lat AS lat2, t1.lon AS lon1, t2.lon AS lon2
        FROM table2 t2 LEFT OUTER JOIN table1 t1 ON 0
    ), t3_x_t4 AS (
        SELECT t3.*, t4.*, 
        t3.lat AS lat3, t4.lat AS lat4, t3.lon AS lon3, t4.lon AS lon4
        FROM table3 t3 LEFT OUTER JOIN table4 t4 ON 0
      UNION ALL
        SELECT t3.*, t4.*, 
        t3.lat AS lat3, t4.lat AS lat4, t3.lon AS lon3, t4.lon AS lon4
        FROM table4 t4 LEFT OUTER JOIN table3 t3 ON 0
    )
    SELECT t1_x_t2.*, t3_x_t4.* FROM t1_x_t2 LEFT OUTER JOIN t3_x_t4 ON 0
    UNION ALL
    SELECT t1_x_t2.*, t3_x_t4.* FROM t3_x_t4 LEFT OUTER JOIN t1_x_t2 ON 0
)

Answer 2

使用pivot() （如果您的dbms支持）导出结果的一种方法可能如下所示。

select a,b,c,d,e,f,g,h,lon,lat
from (
select 'a' as columnName1, a as val1,'b' as ColumnName2, b as val2, lon,lat
from table1
union all
select 'c',c,'d', d, lon,lat
from table2
union all
select 'e',e,'f', f, lon,lat
from table3
union all
select 'g',g,'h', h, lon,lat
from table4
) t1
PIVOT (MAX(val1)
      FOR columnName1 IN (a,c,e,g)) as Pivot1
PIVOT (MAX(val2)
      FOR columnName2 IN (b,d,f,h)) AS Pivot2;

<强>结果：

+------+------+------+------+------+------+------+------+-----+-----+
|  a   |  b   |  c   |  d   |  e   |  f   |  g   |  h   | lon | lat |
+------+------+------+------+------+------+------+------+-----+-----+
| 22   | 33   | NULL | NULL | NULL | NULL | NULL | NULL |  11 |  22 |
| NULL | NULL | 1    | 2    | NULL | NULL | NULL | NULL |  44 |  45 |
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 100 | 101 |
| NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 200 | 201 |
+------+------+------+------+------+------+------+------+-----+-----+

<强> DEMO

P.S。请注意，上述查询适用于sql-Server，因此您可能需要为dbms进行调整。

SQLite：具有四个包含30多列的表的完全外部联接

2 个答案: