lewy zewnętrzny łączący trzy tabele w Oracle

Question

Mam trzy tabele jak pokazano poniżej:

i próbuję dostać wyjście jak poniżej:

Oto co próbowałem do tej pory

SELECT table1.tb1_a, 
CASE WHEN table3.tb3_a IS NOT NULL THEN 
    tb3_b 
    ELSE 'No city for him yet' 
END AS 'City' 
FROM table1 
LEFT OUTER JOIN table2 ON 
    table1.tb1_a = table2.tb2_a 
LEFT OUTER JOIN table3 ON 
    table2.tb2_a = table3.tb3_a 
WHERE table3.tb3_a IN 
    (

    ) 

I teraz walczę o tym, jak wybrać maksymalną wartość kolumny tb3_a

Answer 1

Należy to zrobić co trzeba:

SELECT t1.tb1_a, COALESCE(t3.tb3_a, 'No city for him yet') AS City 
FROM table1 t1 
LEFT JOIN (
    SELECT MAX(tb2_b) AS tb2_b, tb2_a 
    FROM table2 
    GROUP BY tb2_a 
) t2 ON (t2.tb2_a = t1.tb1_a) 
LEFT JOIN table3 t3 ON (t3.tb3_a = t3.tb2_b); 

Kluczowym punktem jest pogląd in-line w środku, gdzie tworzymy rodzaj wirtualnego tabela zawierająca maksymalną wartość tb2_b dla każdego tb2_a. Możemy wtedy dołączyć do tego, aby osiągnąć pożądany rezultat.

Answer 2

Coś jak to powinno działać

select tb1_a, nvl(max(city), 'no city for him yet') thecity 
from etc 
group by tbl_a 

Answer 3

Myślę, że chcesz to zrobić z funkcji analitycznych. Oto jak:

SELECT tb1_a, coalesce(tb3_b, 'No city for him yet') as City 
from (select table1.tb1_a, tb3_b, 
      ROW_NUMBER() over (partition by table1.tbl1_a order by tb3_a desc) as seqnum 
     FROM table1 LEFT OUTER JOIN 
      table2 
      ON table1.tb1_a = table2.tb2_a LEFT OUTER JOIN 
      table3 
      ON table2.tb2_a = table3.tb3_a 
    ) t 
where seqnum = 1 

używa row_number() określić ostatni wpis w table3. Jest to wybrane przez klauzulę where seqnum = 1.

Answer 4

spróbować, nadzieję, że pomoże

with table_1 as 
(select 10 as tb1_a, 'John' as tb1_b 
    from dual 
    union all 
    select 20 as tb1_a, 'John1' as tb1_b 
    from dual 
    union all 
    select 30 as tb1_a, 'John2' as tb1_b from dual), 
table_2 as 
(select 10 as tb2_a, 100 as tb2_b 
    from dual 
    union all 
    select 10 as tb2_a, 1000 as tb2_b 
    from dual 
    union all 
    select 10 as tb2_a, 10000 as tb2_b 
    from dual 
    union all 
    select 20 as tb2_a, 200 as tb2_b 
    from dual 
    union all 
    select 20 as tb2_a, 2000 as tb2_b 
    from dual 
    union all 
    select 20 as tb2_a, 20000 as tb2_b from dual), 
table_3 as 
(select 100 as tb3_a, 'City1' as tb3_b 
    from dual 
    union all 
    select 1000 as tb3_a, 'City10' as tb3_b 
    from dual 
    union all 
    select 10000 as tb3_a, 'City100' as tb3_b 
    from dual 
    union all 
    select 200 as tb3_a, 'City2' as tb3_b 
    from dual 
    union all 
    select 2000 as tb3_a, 'City20' as tb3_b 
    from dual 
    union all 
    select 20000 as tb3_a, 'City200' as tb3_b from dual) 

select user_id, city 
    from (select u.tb1_a as user_id, 
       nvl(c.tb3_b, 'No city') as city, 
       rank() over(partition by u.tb1_a order by tb3_a) as city_rank 
      from table_1 u, table_2 u_c, table_3 c 
     where u.tb1_a = u_c.tb2_a(+) 
      and u_c.tb2_b = c.tb3_a(+)) t1 
where city_rank = 1 

Answer 5

to działa:

http://sqlfiddle.com/#!4/7ba1c/12

SELECT table1.tb1_a, 
     CASE WHEN table3.tb3_a IS NOT NULL THEN tb3_b 
      ELSE 'No city for him yet' 
     END AS City, 
table2.*, table3.* 
FROM table1 
LEFT OUTER JOIN table2 ON table1.tb1_a = table2.tb2_a 
LEFT OUTER JOIN table3 ON table2.tb2_b = table3.tb3_a 
where tb2_b = (select max(tb2_b) 
       from table2 t22 
       where t22.tb2_a = tb1_a) 
or tb2_b is null 
order by 1 

Myślę, że klauzula gdzie wyjaśnia się