How just return one row of a one to many join..

Question

So I have a one to many join where the SMOPERATOR table has data I need however it has a couple of rows that match the JOIN condition in there. I just need to return one row. I think this can be accomplished with a subquery in the join however have not been able to come up with the right syntax to do so.
So:
SELECT "NUMBER" as danumber,
NAME,
SMINCREQ.ASSIGNMENT,
SMOPERATOR.PRIMARY_ASSIGNMENT_GROUP,
SMOPERATOR.WDMANAGERNAME,
SMINCREQ.owner_manager_name,
SMINCREQ.subcategory, TO_DATE('01-'||TO_CHAR(open_time,'MM-YYYY'),'DD-MM-YYYY')MONTHSORT,
(CASE WHEN bc_request='f' THEN 'IAIO'
WHEN (bc_request='t' and substr(assignment,1,3)<>'MTS') THEN 'RARO'
WHEN (bc_request='t' and substr(assignment,1,3)='MTS') THEN 'M'
ELSE 'U' end) as type
from SMINCREQ
left outer join SMOPERATOR on SMINCREQ.assignment=SMOPERATOR.primary_assignment_group
WHERE SMINCREQ.owner_manager_name=:P170_SELECTION and SMOPERATOR.wdmanagername=:P170_SELECTION
AND open_time BETWEEN to_date(:P170_SDATEB,'DD-MON-YYYY') AND to_date(:P170_EDATEB,'DD-MON-YYYY')
AND
(bc_request='f' and subcategory='ACTIVATION' and related_record<>'t')
OR
(bc_request='f' and subcategory<>'ACTIVATION')
OR
(bc_request='t' and substr(assignment,1,3)<>'MTS')
order by OPEN_TIMe

Answer 1

Hi,
This sounds like a Top-N Query , where you pick N items (N=1 in this case) off the top of an orderded list. I think you want a separate ordered list for each assignment; the analytic ROW_NUMBER function does that easily.
Since you didn't post CREATE TABLE and INSERT statements for your sample data, I'll use tables from the scott schema to show how this is done.
Say you have a query like this:
SELECT       d.dname
,       e.empno, e.ename, e.job, e.sal
FROM       scott.dept  d
JOIN       scott.emp   e  ON   d.deptno = e.deptno
ORDER BY  dname
;which produces this output:
DNAME               EMPNO ENAME      JOB              SAL
ACCOUNTING           7934 MILLER     CLERK           1300
ACCOUNTING           7839 KING       PRESIDENT       5000
ACCOUNTING           7782 CLARK      MANAGER         2450
RESEARCH             7876 ADAMS      CLERK           1100
RESEARCH             7902 FORD       ANALYST         3000
RESEARCH             7566 JONES      MANAGER         2975
RESEARCH             7369 SMITH      CLERK            800
RESEARCH             7788 SCOTT      ANALYST         3000
SALES                7521 WARD       SALESMAN        1250
SALES                7844 TURNER     SALESMAN        1500
SALES                7499 ALLEN      SALESMAN        1600
SALES                7900 JAMES      CLERK            950
SALES                7698 BLAKE      MANAGER         2850
SALES                7654 MARTIN     SALESMAN        1250Now say you want to change the query so that it only returns one row per department, like this:
DNAME               EMPNO ENAME      JOB              SAL
ACCOUNTING           7782 CLARK      MANAGER         2450
RESEARCH             7876 ADAMS      CLERK           1100
SALES                7499 ALLEN      SALESMAN        1600where the empno, ename, job and sal columns on each row of output are all taken from the same row of scott.emp, though it doesn't really matter which row that is.
One way to do it is to use the analytic ROW_NUMBER function to assign a sequence of unique numbers (1, 2, 3, ...) to all the rows in each department. Since each sequence startw with 1, and the numbers are unique within a department, there will be exactly one row per departement that was assigned the numebr 1, and we''ll display that row.
Here's how to code that:
WITH     got_r_num     AS
     SELECT     d.dname
     ,     e.empno, e.ename, e.job, e.sal
     ,     ROW_NUMBER () OVER ( PARTITION BY  d.dname
                               ORDER BY          e.ename
                       )         AS r_num
     FROM     scott.dept  d
     JOIN     scott.emp   e  ON   d.deptno = e.deptno
SELECT       dname
,       empno, ename, job, sal
FROM       got_r_num
WHERE       r_num     = 1
ORDER BY  dname
;Notice that he sub-query got_r_num is almost the same as the original query; only it has one additional column, r_num, in the SELECT clause, and the sub-qeury does not have an ORDER BY clause. (Sub-queries almost never have an ORDER BY clause.)
The ROW_NUMBER function must have an ORDER BY clause. In this example, I used "ORDER BY ename", meaning that, within each department, the row with the first ename (in sort order) will get r_num=1. You can use any column, or expression, or expressions in the ORDER BY clause. You muight as well use something consistent and predictable, like ename, but if you really wanted arbitrary numbering you could use a constant in the analytic ORDER BY clause, e.g. "ORDER BY NULL".