Get name from two ids by joining tables (SQL) - sql

Question

First table
MEMO_ID1
MEMO_ID2
UPDATED_BY
1
2
Bob
Second table
MEMO_ID1
MEMO_NAME
1
UD
2
LD
Result table I want:
MEMO_ID1
MEMO_ID2
UPDATED_BY
UD
LD
Bob
SELECT u.MEMO_ID1, u.MEMO_ID2, u.UPDATED_BY
FROM USER u;
How can I join the user and memo tables to get the names of two different IDs?

Answer 1

try with below:
select t2.MEMO_ID1,t2.MEMO_NAME,t11.UPDATED_BY
from table2 t2
join table1 t11 on t2.MEMO_ID1=t11.MEMO_ID1
join table1 t12 on t2.MEMO_ID1=t12.MEMO_ID2
where t11.UPDATED_BY=t12.UPDATED_BY

Answer 2

You didn't mentioned a database engine you're using so I will do all the stuff with T-SQL. Other DB engines have its own functions for pivoting data
In order to join data from table1 with table2 you need to pivot table1.
For pivoting this data you have 2 ways:
one option is using UNPIVOT
SELECT updated_by, pvt_id
FROM (
SELECT memo_id1, memo_id2, updated_by
FROM t1
) pvt
UNPIVOT (pvt_id FOR col_names IN (memo_id1, memo_id2)) AS unpvt
and another one is UNION data like that
SELECT memo_id1, updated_by
FROM t1
UNION
SELECT memo_id2, updated_by
FROM t1
Now you can join this data with table2 and pivot result back
WITH source AS
(
SELECT updated_by, pvt_id
FROM
(
SELECT memo_id1
,memo_id2
,updated_by
FROM t1
) pvt UNPIVOT (pvt_id FOR col_names IN (memo_id1, memo_id2)) AS unpvt
),
r1 AS
(
SELECT *
FROM source s
LEFT JOIN t2
ON t2.memo_id = s.pvt_id
)
SELECT updated_by, [1] AS [memo1], [2] AS [memo2]
FROM
(
SELECT updated_by, memo_name, memo_id
FROM r1
) pvt
PIVOT (MIN(memo_name) for memo_id IN ([1] , [2])) AS pvt2;
or the same with UNION and PIVOT
WITH source (m_id, updated_by) AS
(
SELECT memo_id1, updated_by
FROM t1 union
SELECT memo_id2, updated_by
FROM t1
),
r1 AS
(
SELECT *
FROM source
LEFT JOIN t2
ON t2.memo_id = t1_data.m_id
)
SELECT updated_by,[1] AS [memo1],[2] AS [memo2]
FROM
(
SELECT updated_by, memo_name, memo_id
FROM r1
) pvt
PIVOT (MIN(memo_name) for memo_id IN ([1], [2])) AS pvt2;
Even a bit simpler solution if you really need to solution on the data of your example without extensibility
with source (m_id, updated_by) as (
select memo_id1, updated_by
from t1
union
select memo_id2, updated_by
from t1
)
select s.updated_by, min(t2.memo_name) [memo1], max(t2.memo_name) [memo2]
from source s
LEFT JOIN t2 on t2.memo_id = s.m_id
group BY s.updated_by
;