How do I limit the number of rows returned by this LEFT JOIN to one?

Question

So I think I\'ve seen a solution to this however they are all very complicated queries. I\'m in oracle 11g for reference.

What I have is a simple one to many join w

Answer 1

I think what you need is to use GROUP BY on the ASSIGNMENT_GROUP field.

http://www.w3schools.com/sql/sql_groupby.asp

Answer 2

If oracle supports row number (partition by) you can create a sub query selecting where row equals 1.

SELECT * FROM table1
LEFT JOIN
(SELECT *
FROM   (SELECT *,
           ROW_NUMBER()
             OVER(PARTITION BY assignmentgroup ORDER BY assignmentgroup) AS Seq
    FROM  table2) a
WHERE  Seq = 1) v
ON assignmet = v.assignmentgroup

Answer 3

In MySQL you could just GROUP BY ASSIGNMENT and be done. Oracle is more strict and refuses to just choose (in an undefined way) which values of the three rows to choose. That means all returned columns need to be part of GROUP BY or be subject to an aggregat function (COUNT, MIN, MAX...)

You can of course choose to just don't care and use some aggregat function on the returned columns.

select TICKET_ID, ASSIGNMENT, MAX(MANAGER_NAME), MAX(USER)
from T1
left join T2 on T1.ASSIGNMENT=T2.ASSIGNMENT_GROUP
group by TICKET_ID, ASSIGNMENT

If you do that I would seriously doubt that you need the JOIN in the first place.

MySQL could also help with GROUP_CONCAT in the case that you want a string concatenation of group values for a column (humans often like that), but with Oracle that is staggeringly complex.

Using a subquery as already suggested is an option, look here for an example. It also allows you to sort the subquery before selecting the top row.

Answer 4

In Oracle, if you want 1 result, you can use the ROWNUM statement to get the first N values of a query e.g.:

SELECT *
FROM TABLEX
WHERE
ROWNUM = 1 --gets the first value of the result

The problem with this single query is that Oracle never returns the data in the same order. So, you must oder your data before use rownum:

SELECT *
FROM
    (SELECT * FROM TABLEX ORDER BY COL1)
WHERE
ROWNUM = 1

For your case, looks like you only need 1 result, so your query should look like:

SELECT *
FROM
    TABLE1 T1
    LEFT JOIN 
    (SELECT *
    FROM TABLE2 T2 WHERE T1.ASSIGNMENT = T2.ASSIGNMENT_GROUP
    AND
    ROWNUM = 1) T3 ON T1.ASSIGNMENT = T3.ASSIGNMENT_GROUP

Answer 5

You could do something like this.

SELECT t1.ticket_id, 
       t1.assignment,
       t2.manager_name,
       t2.user
  FROM table1 t1
       LEFT OUTER JOIN (SELECT manager_name,
                               assignment_group,
                               user,
                               row_number() over (partition by assignment_group
                                                    --order by <<something>>
                                                 ) rnk
                          FROM table2) t2
                     ON (    t1.assignment = t2.assignment_group
                         AND t2.rnk = 1 )

This partitions the data in table2 by assignment_group and then arbitrarily ranks them to pull one arbitrary row per assignment_group. If you care which row is returned (or if you want to make the row returned deterministic) you could add an ORDER BY clause to the analytic function.

Answer 6

you can use subquery - select top 1