如何查询 MS-Access 中只有最后一个明细行的主明细结果？

Question

I have the following master table

.------------------.
| id | parent_name | 
.------------------.
| 1  | Mike        |
| 2  | Sarah       |
| 3  | Danial      |
| 4  | Alex        |
.------------------.

And have the following child-table details:

.------------------------------------------.
| id | parent_id | child_name | birth year |
.------------------------------------------.
| 1  |   1       | mandy      |  2000      |
| 2  |   1       | mark       |  2003      |
| 3  |   1       | mathew     |  2005      |
| 4  |   2       | sandy      |  1998      |
| 5  |   2       | sharon     |  2006      |
| 6  |   3       | david      |  2001      |
.------------------------------------------.

In the example above, I delibretaly choose names of children with the first letter matching their parents' names just to make it easier to understand the relationship, even though each child is connected to his/her parent using the parent_id .

What I would like to have is a list of all parents (4 rows) and to have a matching 4 rows from the children table, selecting only the last born child of each respectful parent.

.-------------------------------.
| id | parent_name | last_child |
.-------------------------------.
| 1  | Mike        | mathew     |
| 2  | Sarah       | sharon     |
| 3  | Danial      | david      |
| 4  | Alex        | (null)     |
.-------------------------------.

In oracle, this is easy:

SELECT
   p.id,
   p.parent_name,
   c.child_name last_child
FROM
   parents_table p,
   children_table c
WHERE
        p.id = c.parent_id
   AND  c.birth_year = (SELECT MAX(birth_year) FROM children_table where parent_id = p.id)

But I am struggling to generate the same result in MS Access.. MS Access does not accept sub-queries (for select the child having the maximum birth year for the same parent).

Is there a better way to get the result in MS Access?

Answer 1

Access certainly does support subqueries, but you're using a crossjoin, so you will never get a null there.

Instead, left join and perform a subquery in the FROM clause. Your query would fail identically in Oracle, by the way. There are no relevant differences between Access and Oracle here.

SELECT
   p.id,
   p.parent_name,
   c.child_name last_child
FROM
   parents_table p
   LEFT JOIN
    (
        SELECT *
        FROM children_table c
        WHERE c.birth_year = (SELECT MAX(c2.birth_year) FROM children_table c2 WHERE c2.parent_id = c.parent_id)
    ) c
    ON p.id = c.parent_id

Access sometimes performs better with an EXISTS , so a rewrite to that would be:

SELECT
   p.id,
   p.parent_name,
   c.child_name last_child
FROM
   parents_table p
   LEFT JOIN
    (
        SELECT *
        FROM children_table c
        WHERE EXISTS(SELECT 1 FROM children_table c2 WHERE c2.parent_id = c.parent_id HAVING c.birth_year = MAX(c2.birth_year))
    ) c
    ON p.id = c.parent_id

Answer 2

If you just want the child's name, you can use a correlated subquery:

select p.*,
       (select top 1 child_name
        from children_table as ct
        where ct.parent_id = p.id
        order by ct.birth_year desc, ct.child_name
       ) as youngest_child_name
from parents_table as p;

This can take advantage of an index on children_table(parent_id, birth_year desc, child_name) . With the index, I would expect this to be quite fast.