SQL Update Query - Reversing a reference

I'm new to SQL, so this isn't too complicated.

I have two tables, with currently no Foreign Key Constraint implemented:

 Table Foo
============
ID  |  BarID
------------
1   |  NULL
2   |  NULL
3   |  NULL

and

 Table Bar
============
ID  |  FooID
------------
101 |  1
102 |  2
103 |  3

As you can see, the Bar table rows know which Foos they relate to, but the Foo table rows do not know which Bars they relate to. I want to switch this around, so I get:

 Table Foo
============
ID  |  BarID
------------
1   |  101
2   |  102
3   |  103

I want a SQL query that will set te BarID on the Foo table. In pseudo code:

for each Foo in FooTable:
  Bar = select B开发者_Python百科ar from BarTable where Bar.FooID == Foo.ID
  set Foo.BarID = Bar.ID
  set Bar.FooID = NULL

Thanks

EDIT: I'm using Microsoft SQL Server 2008

update foo set
BarID = (select ID from Bar where FooId = Foo.ID);

It seems you want to clear out references from Bar to Foo. If so, do this:

update Bar set FooID = null;

UPDATE Foo SET BarId=(SELECT Id FROM BAR WHERE Fooid=foo.id)

UPDATE Foo 
JOIN Bar ON Bar.FooID = Foo.ID
SET Foo.BarID = Bar.ID

This should do it

UPDATE Foo
USING Bar
SET BarID=Bar.ID
WHERE Foo.ID = Bar.FooID;