How do I insert data from one table to another when there is unequal number of rows in one another? - sql

Question

I have a table named People that has 19370 rows with playerID being the primary column. There is another table named Batting, which has playerID as a foreign key and has 104324 rows.
I was told to add a new column in the People table called Total_HR, which is included in the Batting table. So, I have to insert that column data from the Batting table into the People table.
However, I get the error:
Msg 515, Level 16, State 2, Line 183 Cannot insert the value NULL into
column 'playerID', table 'Spring_2019_BaseBall.dbo.People'; column
does not allow nulls. INSERT fails. The statement has been terminated.
I have tried UPDATE and INSERT INTO SELECT, however got the same error
insert into People (Total_HR)
select sum(HR) from Batting group by playerID
I expect the output to populate the column Total_HR in the People table using the HR column from the Batting table.

Answer 1

You could use a join
BEGIN TRAN
Update People
Set Total_HR = B.HR_SUM
from PEOPLE A
left outer join
(Select playerID, sum(HR) HR_SUM
from Batting
group by playerID) B on A.playerID = B.playerID
Select * from People
ROLLBACK
Notice that I've put this code in a transaction block so you can test the changes before you commit

Answer 2

From the error message, it seems that playerID is a required field in table People.
You need to specify all required fields of table People in the INSERT INTO clause and provide corresponding values in the SELECT clause.
I added field playerID below, but you might need to add additional required fields as well.
insert into People (playerID, Total_HR)
select playerID, sum(HR) from Batting group by playerID
It is strange, however, that you want to insert rows in a table that should already be there. Otherwise, you could not have a valid foreign key on field playerID in table Batting... If you try to insert such rows from table Batting into table People, you might get another error (violation of PRIMARY KEY constraint)... Unless... you are creating the database just now and you want to populate empty table People from filled/imported table Batting before adding the actual foreign key constraint to table People. Sorry, I will not question your intentions. I personally would consider to update the query somewhat so that it will not attempt to insert any rows that already exist in table People:
insert into People (playerID, Total_HR)
select Batting.playerID, sum(Batting.HR)
from Batting
left join People on People.playerID = Batting.playerID
where People.playerID is null and Batting.playerID is not null
group by playerID

Answer 3

You need to calculate the SUM and then join this result to the People table to bring into the same rows both Total_HR column from People and the corresponding SUM calculated from Batting.
Here is one way to write it. I used CTE to make is more readable.
WITH
CTE_Sum
AS
(
SELECT
Batting.playerID
,SUM(Batting.HR) AS TotalHR_Src
FROM
Batting
GROUP BY
Batting.playerID
)
,CTE_Update
AS
(
SELECT
People.playerID
,People.Total_HR
,CTE_Sum.TotalHR_Src
FROM
CTE_Sum
INNER JOIN People ON People.playerID = CTE_Sum.playerID
)
UPDATE CTE_Update
SET
Total_HR = TotalHR_Src
;