The statement reads:
"... , if you write an UPDATE statement with a table A in the UPDATE clause and a table B (but not A) in the FROM clause, you get an implied cross join between A and B."
Question: Could you possibly make #A and #B to demo the effect of this statement?
Based on above, it seems the query will be:
Set Age_InA = Age_InB
Feel free to make your own SQL to better illustrates it. BTW, the book I am studying is using the following example:
SET MyCustomers.country = SRC.country,
MyCustomers.postalcode = SRC.postalcode
FROM Sales.Customers AS SRC
-- WHERE MyCustomers.custid = SRC.custid; -- with WHERE clause this will be like inner join.