SQLite: full outer join with four tables

问题

EDIT: An extension to this question can be found here
I want to join four different tables with SQLite which have just two columns in common. Please take a look at following example

+--------+---+-----+-----+
| table1 |   |     |     |
+--------+---+-----+-----+
| a      | b | lon | lat |
+--------+---+-----+-----+
| 1      | 2 | 111 | 222 |
+--------+---+-----+-----+
+--------+---+-----+-----+
| table2 |   |     |     |
+--------+---+-----+-----+
| c      | d | lon | lat |
+--------+---+-----+-----+
| 3      | 4 | 333 | 444 |
+--------+---+-----+-----+
+--------+---+-----+-----+
| table3 |   |     |     |
+--------+---+-----+-----+
| e      | f | lon | lat |
+--------+---+-----+-----+
| 5      | 6 | 555 | 666 |
+--------+---+-----+-----+
+--------+---+-----+-----+
| table4 |   |     |     |
+--------+---+-----+-----+
| g      | h | lon | lat |
+--------+---+-----+-----+
| 7      | 8 | 777 | 888 |
+--------+---+-----+-----+

The tables are not connected by any foreign key. Also, the lon/lat values are different for every row. An optimal output would be:

+------+------+------+------+------+------+------+------+-----+-----+
|  a   |  b   |  c   |  d   |  e   |  f   |  g   |  h   | lon | lat |
+------+------+------+------+------+------+------+------+-----+-----+
| None | None | 3    | 4    | None | None | None | NOne | 333 | 444 |
| 1    | 2    | None | None | None | None | None | None | 111 | 222 |
| None | None | None | None | 5    | 6    | None | None | 555 | 666 |
| None | None | None | None | None | None | 7    | 8    | 777 | 888 |
+------+------+------+------+------+------+------+------+-----+-----+

Again, this is my end goal - lat/lon values are not interconnected between the tables:
- a | b | c | .. | lat | lon | instead of
- a | b | c | .. | table1.lat | table2.lat | ...
Appreciate your help!
My current code

-- First two tables
CREATE VIEW ab AS
SELECT * FROM table1 LEFT JOIN table2 ON ???
UNION ALL
SELECT * FROM table2 LEFT JOIN table1 ON ?? WHERE ?? IS NULL

-- 3rd and 4th table
CREATE VIEW cd AS
SELECT * FROM table3 LEFT JOIN table4 ON ??
UNION ALL
SELECT * FROM table4 LEFT JOIN table3 ON ?? WHERE ?? IS NULL

-- -- JOIN
SELECT * FROM cd LEFT JOIN ab ON ??
UNION ALL
SELECT * FROM cd LEFT JOIN ab ON ?? WHERE ?? IS NULL

回答1:

Assuming that your sample tables has data like below

Table1:

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

Table2:

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

Table3

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

Table4

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

and you want to merge the records, you can use union all.

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

Result:

+------+------+------+------+------+------+------+------+-----+-----+
|  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

来源：https://stackoverflow.com/questions/47666005/sqlite-full-outer-join-with-four-tables

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sql

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join

full-outer-join