将多个表合并在一起，正确的联接类型？

Question

I have more then 10 tables I would like to join together. Here is a basic example of what I need to do with 3 tables.

Input Tables

Table 1
    ID, Time, Field1
    1, Mon, 5
    1, Tue, 6
    1, Wed, 7 

Table 2
    ID, Time, Field1
    1, Tue, 99
    1, Wed, 199
    1, Thu, 299

Table 3
    ID, Time, Field1
    1, Wed, 777
    1, Thu, 888
    1, Fri, 999

Desired Output Table

Table Output
    ID, Time, T1.Field1, T2.Field1, T3.Field1
    1, Mon, 5, NULL, NULL
    1, Tue, 6, 99, NULL
    1, Wed, 7, 199, 777
    1, Thu, NULL, 299, 888
    1, Fri, NULL, NULL, 999

I have tried the following(with 9 tables), and it is not returning. Is this due to run time or have I done the wrong kind of join?

Select 
    id, time, t1.field1, t2.field1, t3.field3
From
    table1 t1
FULL JOIN table2 t2 using (id, time)
FULL JOIN table3 t3 using (id, time);

Answer 1

You need a Left Outer join. This will get all rows in t1, and any matching data in t2, t3 etc, but return NULLS if there is no matching data.

Answer 2

Your query actually seems correct, or at least sane, bar an ambiguous column name and gaps that prevent rows from showing up when rows exist in table3 but not in table1 and table2. So from the third table onward, you'll need to coalesce all over the place:

Select 
    coalesce(t1.id, t2.id, t3.id, ...) as id,
    coalesce(t1.time, t2.time, t3.time, ...) as time,
    t1.field1 as field1,
    t2.field1 as field2, ...
From
    table1 t1
FULL JOIN table2 t2 using (id, time)
FULL JOIN table3 t3 on t3.id = coalesce(t1.id, t2.id)
                   and t3.time = coalesce(t1.time, t2.time)
FULL JOIN ...

If you want to fill the holes in addition, look into generate_series() -- and if you end up using the latter, left join and no coalesce will be fine.

Also, seriously revisit that schema, and avoid joining the above mess with anything -- seq scans all over are guaranteed if you do.

Answer 3

How about a pseudo table as the master being a complete list of unique id/time combinations and LEFT JOINing off that to ensure all other tables can be represented?

SELECT m.id,m.time,t1.field,t2.field,t3.field
FROM (SELECT DISTINCT id,time FROM (
    SELECT id,time FROM t1 
    UNION 
    SELECT id,time FROM t2
    UNION
    SELECT id,time FROM t3)m) m
LEFT JOIN t1 ON t1.id=m.id AND t1.time=m.time
LEFT JOIN t2 ON t2.id=m.id AND t2.time=m.time
LEFT JOIN t3 ON t3.id=m.id AND t3.time=m.time

Efficiency may not be the best.