Need SQL outer join help with 3 tables

join;oracle

See below example. ID_1 and ID_A are primary keys for table_1 and table_3 respectively. Table_2, the primary key is ID_1 and ID_A. ID_A is joined to one and only one ID_1 via table_2 due to business rules.

I need a query that will return all rows from table_1 where the value_1 = 11 and joined to that, I need only the rows from table_3 that have a value_a = 'a' if it joins to a row in table_1 that has a value_1 = 11.

Please see results required. Note there is no row 

|1 | a |

in the result set because value_a for id_a = 'b';

I've tried a left outer join but all ID_1 are returned. I've also tried CTE, to no avail. Here's one SQL, I tried.

Select ID_1, Value_1, ID_A, VALUE_A
   From TABLE_1 t1
   Join TABLE_2 t2 on t1.ID_1 = t2.ID_1
   Left Outer Join TABLE_3 t3 on t2.ID_A = t3.ID_A;

I've also tried using a temporary table which include only the results from TABLE_1 that I need, but the left outer join using the temp table still returns the above mentioned row.

The DB is Oracle, but I prefer a standard solution. Also, performance is a consideration, given that TABLE_1 has over 200 million rows and TABLE_3 has about 4 times that many.

TABLE_1

| ID_1 | Value_1  |
-------------------
| 1    | 11       |
| 2    | 12       |
| 3    | 11       |
| 4    | 13       |

TABLE_2

| ID_1 | ID_A  |
----------------
| 1    | a     |
| 1    | b     |
| 3    | c     |
| 4    | d     |

TABLE_3

| ID_A | Value_A |
------------------
| a    | b       |
| b    | a       |
| c    | e       |
| d    | a       |

Results Required

| ID_1 | ID_A    |  Value_1  | Value_A |
----------------------------------------
| 1    | b       |    11     |    a    |
| 3    | <NULL>  |    11     | <NULL>  |

Best Answer

First, join TABLE_2 and TABLE_3 using an inner join and additionally filtering on Value_A = 'a':

SELECT
  t2.ID_1,
  t3.ID_A,
  t3.Value_A
FROM
  TABLE_2 t2
  INNER JOIN TABLE_3 t3 ON t2.ID_A = t3.ID_A
WHERE
  t3.Value_A = 'a'

This will give you the following result set:

ID_1  ID_A  Value_A
----  ----  -------
1     b     a
4     d     a

Now use the above as a derived table and join it, using an outer join this time, to TABLE_1, additionally filtering the results on Value_1 = 11:

SELECT
  t1 .ID_1,
  t23.ID_A,
  t1 .Value_1,
  t23.Value_A
FROM
  TABLE_1 t1
  LEFT JOIN
  (
    SELECT
      t2.ID_1,
      t3.ID_A,
      t3.Value_A
    FROM
      TABLE_2 t2
      INNER JOIN TABLE_3 t3 ON t2.ID_A = t3.ID_A
    WHERE
      t3.Value_A = 'a'
  ) t23 ON t1.ID_1 = t23.ID_1
WHERE
  t1.Value_1 = 11
;

That will give you the output you want:

ID_1  ID_A  Value_1  Value_A
----  ----  -------  -------
1     b     11       a
3     NULL  11       NULL

However, nesting a query is not the only way to solve your problem – you can also use a nested join, which is much more concise:

SELECT
  t1.ID_1,
  t3.ID_A,
  t1.Value_1,
  t3.Value_A
FROM
  TABLE_1 t1
  LEFT JOIN
    TABLE_2 t2
    INNER JOIN TABLE_3 t3 ON t2.ID_A = t3.ID_A AND t3.Value_A = 'a'
  ON t1.ID_1 = t2.ID_1
WHERE
  t1.Value_1 = 11
;

The last query implements exactly the same logic as the previous query: first, tables TABLE_2 and TABLE_3 are joined and the result is filtered, then it is joined to TABLE_1 and the final set is filtered again. Some people also add brackets around a nested join:

FROM
  TABLE_1 t1
  LEFT JOIN
  (
    TABLE_2 t2
    INNER JOIN TABLE_3 t3 ON t2.ID_A = t3.ID_A AND t3.Value_A = 'a'
  )
  ON t1.ID_1 = t2.ID_1

to make it clearer (perhaps both for themselves and for future maintainers) that the nested join takes place before the outer-level one, logically, although the syntax is unambiguous enough without them.

Nevertheless, many people find it confusing even with brackets, and if you find yourself struggling with it as well, there is another option – right outer join:

SELECT
  t1.ID_1,
  t3.ID_A,
  t1.Value_1,
  t3.Value_A
FROM
  TABLE_2 t2
  INNER JOIN TABLE_3 t3 ON t2.ID_A = t3.ID_A AND t3.Value_A = 'a'
  RIGHT JOIN TABLE_1 t1 ON t1.ID_1 = t2.ID_1
WHERE
  t1.Value_1 = 11
;

Again, the logical sequence of events specified by the FROM clause repeats that of the derived table solution: a join between TABLE_2 and TABLE_3, filtered, is followed by a join with TABLE_1. The different syntax does not alter the outcome and the results produced still match your requirements.

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