SQL/PostgreSQL: How to select limited amount of rows of different types based on limits stored in a different table? - sql

Question

I have a table (table 1) where the first column is the key and the second column contains elements of different types. In table 1, there's three types (type A, B, C) but the actual database have many more types.
Table.1. A minimal example.
_________________
| | |
|_KEY| attribute |
|____|___________|
|k1 | A |
|k2 | A |
|k3 | B |
|k4 | C |
|k5 | C |
|____|___________|
From table 1; I am interested in retrieving only a limited amount of elements from each type. The limited amount of elements of a given type is provided by table 2, in which the elements type is the key of the table (_element).
To clarify; The limited amount of elements of type A to obtain from table 1. in this minimal example is 1. Likewise, for type B it is 2 and for type C it is 1.
Table 2. Limits of item to obtain for each type in table 1.
____________________
| _Element | Limit |
|----------|-------|
| A | 1 |
| B | 2 |
| C | 1 |
|__________|_______|
Finally, the elements should be retrieved from table 1 from top to bottom.
Thanks for any help and/or pointers / gus.
P.S.
For the above minimal example, the expected output would be
___________________
| Key| Attribute |
|____|____________|
| k1 | A |
| k3 | B |
| K4 | C |
|____|____________|
Since there only exists 1 C attribute for this particular minimal example. Note that if there would have existed, say 5 elements of type C then the follow table would have been obtained instead (since the limited amount of C elements is 2)
___________________
| Key| Attribute |
|____|____________|
| k1 | A |
| k3 | B |
| K4 | C |
|_k5 | C |
|____|____________|

Answer 1

You can always do it with a union.
select top (SELECT Limit FROM Table2 WHERE _Element='A') * from Table1
WHERE attribute = A
UNION ALL
select top (SELECT Limit FROM Table2 WHERE _Element='B') * from Table1
WHERE attribute = B
UNION ALL
select top (SELECT Limit FROM Table2 WHERE _Element='C') * from Table1
WHERE attribute = C
Or using row_number:
with cte as (SELECT _Key,
attribute,
ROW_NUMBER() OVER (Partition by attribute Order by _Key ASC) as rowno
From Table1)
SELECT * FROM cte
LEFT JOIN Table2 on Table2.Element = Table1.attribute
WHERE rowno >= Limit

Answer 2

I truly like the power of PostgreSQL arrays. So
select
table2._element,
unnest((array_agg(table1._key order by table1._key desc)[1:table2.limit])) as _key
from
table1 join table2 on (table1.attribute = table2._element)
group by
table2._element, table2.limit
where in the second field of the query:
array_agg(table1._key order by table1._key desc) - collects values into array in the specified order (note that order by table1._key desc is just for example and you might to skip it or to specify another one),
(...)[1:table2.limit] - returns array elements from 1 to table2.limit,
unnest(...) - unwraps previous result to rows.