在Oracle中限制LISTAGG结果

Question

I'm trying to limit one of my columns in my SQL query that uses LISTAGG to only group the first 3 rows into a single column.

For instance:

Table
-----
Name   Orders
---------------
Joe    Joe_Order1
Joe    Joe_Order2
Joe    Joe_Order3
Joe    Joe_Order4
Joe    Joe_Order5
Joe    Joe_Order6
Mark   Mark_Order1
Mark   Mark_Order2
Mark   Mark_Order3
Mark   Mark_Order4

Have it return the following...

Name   Recent_Orders
-----------------------------
Joe    Joe_Order1, Joe_Order2, Joe_Order3
Mark   Mark_Order1, Mark_Order2, Mark_Order3

I'm able to concatenate the data using listagg however, I'm not entirely sure how to limit the results to the first 3 records.

SELECT NAME, LISTAGG(Orders, ', ') within group(order by Orders)
  as Recent_Orders
FROM
  Order_table
GROUP BY
  NAME

Is this possible with LISTAGG? Any help would be greatly appreciated. Thanks

Answer 1

Apply a row_number in a CTE, then apply the restriction in the WHERE clause

with CTE as
(
select row_number() over(partition by NAME order by Orders) as rn,
       a1.*
from Order_Table a1
)
SELECT NAME, LISTAGG(Orders, ', ') within group(order by Orders)
  as Recent_Orders
FROM
  CTE
WHERE 
  rn <=3
GROUP BY
  NAME

Answer 2

You can do this by enumerating the rows and using case :

SELECT NAME,
       LISTAGG(CASE WHEN seqnum <= 3 THEN Orders END, ', ') WITHIN GROUP (ORDER BY Orders) as Recent_Orders
FROM (SELECT o.*, ROW_NUMBER() OVER (PARTITION BY NAME ORDER BY ?? DESC) as seqnum
      FROM Order_table o
     ) o
GROUP BY NAME;

By default, LISTAGG() ignores NULL values, so this does what you want.

The ?? is the column for specifying the ordering. SQL tables represent unordered sets; there is no "first three" or "last three" unless a column specifies the ordering.