Full Outer Join With Sqlite

FULL OUTER JOIN with SQLite

Yes, see the example on Wikipedia.

SELECT employee.*, department.*
FROM   employee 
       LEFT JOIN department 
          ON employee.DepartmentID = department.DepartmentID
UNION ALL
SELECT employee.*, department.*
FROM   department
       LEFT JOIN employee
          ON employee.DepartmentID = department.DepartmentID
WHERE  employee.DepartmentID IS NULL

Full Outer Join in sqlite on 4 tables

The general construction for a full outer join between two tables A and B in SQLite indeed is:

SELECT ... FROM A LEFT JOIN B ON ...
UNION ALL
SELECT ... FROM B LEFT JOIN A ON ... WHERE A.key IS NULL

Now create a view SocksCaps for the full outer join between Socks and Caps:

CREATE VIEW SocksCaps AS
SELECT ... FROM Socks LEFT JOIN Caps ON ...
UNION ALL
SELECT ... FROM Caps LEFT JOIN Socks ON ... WHERE Socks.color IS NULL

Do the same for Pants and Boxers.

Then treat these views just like tables and do a full outer join with the same construction:

SELECT ... FROM SocksCaps LEFT JOIN PantsBoxers ON ...
UNION ALL
SELECT ... FROM PantsBoxers LEFT JOIN SocksCaps ON ... WHERE SocksCaps.color IS NULL

SQLite: full outer join with four tables

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

SQLite: full outer join with four tables with 30+ columns

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.

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
)

SQLite3 Simulate RIGHT OUTER JOIN with LEFT OUTER JOIN's without being able to change table order

Ignoring column order, x right join y on c is y left join x on c. This is commonly explicitly said. (But you can also just apply the definitions of the joins to your original expression to get subexpressions with the values you want.) You can read the from grammar to see how you can parenthesize or subquery for precedence. Applying the identity we get ts left join (cs left join (p left join cp on x) on y) on z.

Similarly, ignoring column order, x full join y on c is y full join x on c. Expressing full join in terms of left join & right join is a frequently asked duplicate.

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