Query to display one to many result in a single table - sql

Question

Ive been trying to use the GROUP function and also PIVOT but I cannot wrap my head around how to merge these tables and combine duplicate rows. Currently my SELECT statement returns results with duplicate UserID rows but I want to consolidate them into columns.
How would I join TABLE1 and TABLE2 into a new table which would look something like this:
NEW TABLE:
UserID Username ParentID 1 ParentID 2
--------- -------- -------- ----------
1 Dave 1 2
2 Sally 3 4
TABLE1:
UserID Username ParentID
--------- -------- --------
1 Dave 1
1 Dave 2
2 Sally 3
2 Sally 4
Table 2:
ParentID Username
--------- --------
1 Sarah
2 Joe
3 Tom
4 Mark

Answer 1

O r a c l e
The with clause is here just to generate some sample data and, as such, it is not a part of the answer.
After joining the tables you can use LAST_VALUE analytic function with windowing clause to get the next PARENT_ID of the user. That column (PARENT_ID_2) contains a value only within the first row of a particular USER_ID (ROW_NUMBER analytic function). Afterwords just filter out rows where PARENT_ID_2 is empty...
Sample data:
WITH
tbl_1 AS
(
Select 1 "USER_ID", 'Dave' "USER_NAME", 1 "PARENT_ID" From Dual Union All
Select 1 "USER_ID", 'Dave' "USER_NAME", 2 "PARENT_ID" From Dual Union All
Select 2 "USER_ID", 'Sally' "USER_NAME", 3 "PARENT_ID" From Dual Union All
Select 2 "USER_ID", 'Sally' "USER_NAME", 4 "PARENT_ID" From Dual
),
tbl_2 AS
(
Select 1 "PARENT_ID", 'Sarah' "USER_NAME" From Dual Union All
Select 2 "PARENT_ID", 'Joe' "USER_NAME" From Dual Union All
Select 3 "PARENT_ID", 'Tom' "USER_NAME" From Dual Union All
Select 4 "PARENT_ID", 'Mark' "USER_NAME" From Dual
)
Main SQL:
SELECT
*
FROM (
SELECT
t1.USER_ID "USER_ID",
t1.USER_NAME "USER_NAME",
t1.PARENT_ID "PARENT_ID_1",
CASE
WHEN ROW_NUMBER() OVER(PARTITION BY t1.USER_ID ORDER BY t1.USER_ID) = 1
THEN LAST_VALUE(t1.PARENT_ID) OVER(PARTITION BY t1.USER_ID ORDER BY t1.USER_ID ROWS BETWEEN CURRENT ROW AND 1 FOLLOWING)
END "PARENT_ID_2"
FROM
tbl_1 t1
INNER JOIN
tbl_2 t2 ON(t1.PARENT_ID = t2.PARENT_ID)
)
WHERE PARENT_ID_2 Is Not Null
... and the Result ...
-- USER_ID USER_NAME PARENT_ID_1 PARENT_ID_2
-- ---------- --------- ----------- -----------
-- 1 Dave 1 2
-- 2 Sally 3 4
The windowing clause in this answer
ROWS BETWEEN CURRENT ROW AND 1 FOLLOWING
takes curent and next row and returns the value defined by the analytic function (LAST_VALUE) taking care of grouping (PARTITION BY) and ordering of the rows. Regards...

Answer 2

This is mySql ver 5.6. Create a concatenated ParentID using group concat then separate the concatenated ParentID (1,2) and (3,4) into ParentID 1 and Parent ID 2.
SELECT t1.UserID,
t1.Username,
SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(t1.ParentID), ',', 1), ',', -1) AS `ParentID 1`,
SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(t1.ParentID), ',', 2), ',', -1) as `ParentID 2`
FROM TABLE1 t1
INNER JOIN TABLE2 t2 on t1.ParentID = t2.ParentID
GROUP BY t1.UserID
ORDER BY t1.UserID;
Result:
UserID Username ParentID 1 ParentID 2
1 Dave 1 2
2 Sally 3 4