UPDATE FROM in SQL with matching key - sql

Question

How can I UPDATE FROM in SQL? I want to copy the FamilyID from my FamilyGuardian table:
dbo.FamilyGuardian
PupilID FamilyID GuardianID
---------------------------------
1 100002 555
2 100003 556
3 100004 557
4 100005 558
5 100006 559
6 100007 1146
7 100008 561
8 100009 562
9 100010 563
10 100011 564
Into the Guardians table where GuardianID is equal.
dbo.Guardians
GuardianID Name FamilyID
---------------------------------
555 Smith NULL
556 Patel NULL
557 Andrews NULL
558 Brown NULL
559 Abdul NULL
dbo.Guardians after INSERT FROM
GuardianID Name FamilyID
--------------------------------
555 Smith 100002
556 Patel 100003
557 Andrews 100004
558 Brown 100005
559 Abdul 100006

Answer 1

you have to use an UPDATE query
UPDATE T1
SET T1.FamiltyID = T2.FamilyID
FROM dbo.Guardians AS T1 INNER JOIN dbo.FamilyGuardian AS T2
ON T1.GuardianID = T2.GuardianID

Answer 2

There is no such thing as INSERT FROM - you're looking for UPDATE, not INSERT:
Update G
Set FamilyId = F.FamilyId
From FamilyGuardian F
Join Guardian G On G.GuardianId = F.GuardianId

Answer 3

You might use an updateable CTE for this
WITH UpdateableCTE AS
(
SELECT g.FamilyID
,fg.FamilyID AS newValue
FROM dbo.Guardians AS g
INNER JOIN dbo.FamilyGuardian AS fg ON g.GuardianID=fg.GuardianID
)
UPDATE UpdateableCTE SET FamilyID=newValue;
But you must be aware, that this design is - uhm - weak...
What would happen if one Guard is responsible for more than one family? What happens, if the information on one side changes, but the other side does not change?
I think, this should not get into a physically defined column at all. This should be taken with a JOIN whenever you read this data.

Answer 4

you mean an UPDATE, not an INSERT, right?
In that case you should use an structue like:
UPDATE dbo.Guardians
SET FamilyID = (SELECT DISTINCT FamilyID FROM dbo.FamilyGuardian fg WHERE fg.GuardianID = GuardianID )
Notice that the DISTINCT is only necessary when there are more than 1 familyId per guardianId.