SQL Joins . One to many relationship - sql

Question

I have two tables as below
Table 1
-----------------------------------
UserID | UserName | Age | Salary
-----------------------------------
1 | foo | 22 | 33000
-----------------------------------
Table 2
------------------------------------------------
UserID | Age | Salary | CreatedDate
------------------------------------------------
1 | NULL | 35000 | 2015-01-01
------------------------------------------------
1 | 28 | NULL | 2015-02-01
------------------------------------------------
1 | NULL | 28000 | 2015-03-01
------------------------------------------------
I need the result like this.
Result
-----------------------------------
UserID | UserName | Age | Salary
-----------------------------------
1 | foo | 28 | 28000
-----------------------------------
This is just an example. In my real project I have around 6 columns like Age and Salary in above tables.
In table 2 , each record will have only have one value i.e if Age has value then Salary will be NULL and viceversa.
UPDATE :
Table 2 has CreatedDate Column. So i want to get latest "NOTNULL" CELL Value instead of maximum value.

Answer 1

You can get this done using a simple MAX() and GROUP BY:
select t1.userid,t1.username, MAX(t2.Age) as Age, MAX(t2.Salary) as Salary
from table1 t1 join
table2 t2 on t1.userid=t2.userid
group by t1.userid,t1.username
Result:
userid username Age Salary
--------------------------------
1 foo 28 35000
Sample result in SQL Fiddle

Answer 2

Note: I'm giving you the benefit of the doubt that you know what you're doing, and you just haven't told us everything about your schema.
It looks like Table 2 is actually an "updates" table, in which each row contains a delta of changes to apply to the base entity in Table 1. In which case you can retrieve each column's data with a correlated join (technically an outer-apply) and put the results together. Something like the following:
select a.UserID, a.UserName,
coalesce(aAge.Age, a.Age),
coalesce(aSalary.Salary, a.Salary)
from [Table 1] a
outer apply (
select Age
from [Table 2] x
where x.UserID = a.UserID
and x.Age is not null
and not exists (
select 1
from [Table 2] y
where x.UserID = y.UserID
and y.Id > x.Id
and y.Age is not null
)
) aAge,
outer apply (
select Salary
from [Table 2] x
where x.UserID = a.UserID
and x.Salary is not null
and not exists (
select 1
from [Table 2] y
where x.UserID = y.UserID
and y.Id > x.Id
and y.Salary is not null
)
) aSalary
Do note I am assuming you have at minimum an Id column in Table 2 which is monotonically increasing with each insert. If you have a "change time" column, use this instead to get the latest row, as it is better.

Answer 3

To get the latest value based on CreatedDate, you can use ROW_NUMBER to filter for latest rows. Here the partition is based UserID and the other columns, Age and Salary.
SQL Fiddle
;WITH Cte AS(
SELECT
UserID,
Age = MAX(Age),
Salary = MAX(Salary)
FROM(
SELECT *, Rn = ROW_NUMBER() OVER(
PARTITION BY
UserID,
CASE
WHEN Age IS NOT NULL THEN 1
WHEN Salary IS NOT NULL THEN 2
END
ORDER BY CreatedDate DESC
)
FROM Table2
)t
WHERE Rn = 1
GROUP BY UserID
)
SELECT
t.UserID,
t.UserName,
Age = ISNULL(c.Age, t.Age),
Salary = ISNULL(c.Salary, t.Salary)
FROM Table1 t
LEFT JOIN Cte c
ON t.UserID = c.UserID

Answer 4

following query should work(working fine in MSSQL) :
select a.userID,a.username,b.age,b.sal from <table1> a
inner join
(select userID,MAX(age) age,MAX(sal) sal from <table2> group by userID) b
on a.userID=b.userID