SQLite:具有四个包含30多个列的表的完全外部联接 [英] SQLite: full outer join with four tables with 30+ columns

问题描述

此问题是对此的扩展发布
我想用SQLite联接四个不同的表，它们只有两个共同的列.但是，假设有30多个列，即不只是 a-h 列.请看下面的例子

This question is an extension to this post
I want to join four different tables with SQLite which have just two columns in common. However, assume that there are 30+ columns, i.e more than just columns a - h. Please take a look at following example

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

当前解决方案如下

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

DEMO

• 问题:如果我不仅有直到h的a列，而且还有直到z的列，即table1，table2，table3和table4中的许多列，该怎么办->这将非常耗时并且结构不会清楚我是否必须在我的sql语句中到处都写NULL作为[letter]
• @zarruq是一个巨大的帮助，他建议在那种情况下，我可以只使用"UNION ALL"然后使用"PIVOT"将列转换为行
• 但是，我不确定该怎么做.而且，我不知道他100％的意思.
• SQLite不支持数据透视:还有其他建议吗?

• Problem: What if I have not just the columns a until h, but a until z, i.e many many columns in table1, table2, table3, and table4 -> It would be very time-consuming and the structure wouldn't be clear if I had to write everywhere NULL as [letter] in my sql statement
• @zarruq was a huge help and he suggested that In that case I can just use `UNION ALL` and then `PIVOT` to convert columns to rows
• However, I am not sure how to do that. And, I do not know 100% what he means by that.
• SQLite does not support pivot: Any other suggestions?

推荐答案

当您拥有30列以上的列时，我不认为有一种特别整洁的方法.以下是我能做的最好的事情:使用嵌套CTE实现完全外部联接，然后使用

I don't believe there's a particularly neat way of doing this when you have 30+ columns. The following is the best I could do, using nested CTEs to implement full outer joins, then using coalesce to choose the first non-null lat/lon.

仍然需要枚举顶部SELECT语句中的所有30+个字段，但是至少不需要大量的NULL AS ...列表:

It's still required to enumerate all 30+ fields in the top SELECT statement, but at least the massive lists of NULL AS ... aren't needed:

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
)

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