Oracle SQL - Derive multiple rows from column combination of single row

Question

I have this kind of data on a Oracle SQL DB:

personId  lastEvent currentEvent nextEvent
1         null      1            2
1         1         2            3
1         2         3            4
1         3         4            null

For a single row like

personId  lastEvent currentEvent nextEvent
1         null      1            2

I want to extract the two rows extracted by combining together the events columns and categorizing these two rows to the specific currentEvent. For example:

Row #1

personId  lastEvent currentEvent nextEvent
1         null      1            2

Result #1

personId  event1 event2 currentEvent
1         null   1      1
1         1      2      1

Row #2

personId  lastEvent currentEvent nextEvent
1         1         2            3

Result #2

personId  event1 event2 currentEvent
1         1      2      2
1         2      3      2

etc..... I know the existence of the clause CONNECT BY but I'm not really able to figure out the query to obtain these results.

score 1 · Answer 1 · answered Feb 12 '19 at 16:00

Here is one way of doing this.

I didnt use hierarchical query for this though.

create table t(personId int,lastEvent int, currentEvent int, nextEvent int);

insert into t values(1,null,1,2);
insert into t values(1,1,2,3);
insert into t values(1,2,3,4);
insert into t values(1,3,4,null);


    select a.personId,case when b.rn=1 then 
                            a.lastEvent 
                       else a.currentEvent
                   end as event1
       ,case when b.rn=1 then 
                  a.currentEvent 
             else a.nextEvent
         end as event2
       ,case when a.nextEvent is not null then 
                  a.currentEvent
        end as currentEvent       
  from t a
  join (select rownum as rn from all_objects where rownum<=2) b
    on case when a.nextEvent is not null then 2
            else 1
        end >=b.rn
order by 1,4,3   


+----------+--------+--------+--------------+
| PERSONID | EVENT1 | EVENT2 | CURRENTEVENT |
+----------+--------+--------+--------------+
|        1 |     -  |      1 |            1 |
|        1 |      1 |      2 |            1 |
|        1 |      1 |      2 |            2 |
|        1 |      2 |      3 |            2 |
|        1 |      2 |      3 |            3 |
|        1 |      3 |      4 |            3 |
|        1 |      3 |      4 |           -  |
+----------+--------+--------+--------------+

score 1 · Accepted Answer · answered Feb 12 '19 at 21:59

This is not a hierarchical query at all: you merely want to split each input row into two output rows and apply a little logic.

An easy way to perform the split is to CROSS JOIN your table to a row source that has exactly two rows.

E.g.,

with input_data ( personId,  lastEvent, currentEvent, nextEvent) AS
(
SELECT 1, null, 1, 2     FROM DUAL UNION ALL
SELECT 1, 1,    2, 3     FROM DUAL UNION ALL
SELECT 1, 2,    3, 4     FROM DUAL UNION ALL
SELECT 1, 3,    4, null  FROM DUAL )
SELECT personId,
       decode(rn, 1, lastEvent, 2, currentEvent) event1,
       decode(rn, 1, currentEvent, 2, nextEvent) event2,
       currentEvent
from input_data 
CROSS JOIN ( SELECT rownum rn FROM DUAL CONNECT BY ROWNUM <= 2 ) r
ORDER BY personId, currentEvent, rn;

+----------+--------+--------+--------------+
| PERSONID | EVENT1 | EVENT2 | CURRENTEVENT |
+----------+--------+--------+--------------+
|        1 |        |      1 |            1 |
|        1 |      1 |      2 |            1 |
|        1 |      1 |      2 |            2 |
|        1 |      2 |      3 |            2 |
|        1 |      2 |      3 |            3 |
|        1 |      3 |      4 |            3 |
|        1 |      3 |      4 |            4 |
|        1 |      4 |        |            4 |
+----------+--------+--------+--------------+

Thank you very much! Probably I have some gaps in my sql knowledge that need to be fill. — VahnIronSky, Feb 13 '19 at 09:31

Oracle SQL - Derive multiple rows from column combination of single row

2 Answers2