Way to Join only the first record
How Do I in Oracle SQL make Left join between two tables which take only the first record in the right join?
For Example
Table 1:
A 100
B 200
C 300
Table:2
A 900
A 300
The Result will be
A 100 900
B 200 null
C 300 null
Thanks
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I agree you should set up a subquery as table source, which delivers only those rows you want to have joined. As soon as you are able to write up that query, the remainder is easy.
select t1.col1 as t1_value, t2.col1 as t2_value from
(select col1, rownum rn from table1) t1
left join
(select col1, rownum rn from table2
where rownum = 1) t2
on t1.rn = t2.rn;
insert into table1 values(100);
insert into table1 values(200);
insert into table1 values(300);
insert into table2 values(900);
insert into table2 values(300);
SQL> select * from table1;
COL1
----------
100
200
300
SQL> select * from table2;
COL1
----------
900
300
SQL> select col1, rownum rn from table1;
COL1 RN
---------- ----------
100 1
200 2
300 3
SQL> select col1, rownum rn from table2;
COL1 RN
---------- ----------
900 1
300 2
SQL> select t1.col1 as t1_value, t2.col1 as t2_value from
2 (select col1, rownum rn from table1) t1
3 left join
4 (select col1, rownum rn from table2
5 where rownum = 1) t2
6 on t1.rn = t2.rn;
T1_VALUE T2_VALUE
---------- ----------
100 900
300
200
Until we are given a precise and meaningful determination of what constitutes the first row (in both tables), the use of rownum is a readily available option. I could have also used row_number() over (partition by rowid) as rn and it would have produced the same results but, again with deletions and inserts would become skewed.
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Otherwise just use a sub-query as source for the left joined table, which return a single row.