MS SQL Update Set Select

There's a table A, there's data.

Then there's a table B, and the data is also queried

Two tables have foreign key associations

The requirements are as follows:

Update a field in table A to add (all data of type int) to the data in table B as the updated final data

Update set acticount=ISNULL(Acticount,0)+ISNULL(Total,0 ) from the left    join on #libList. ID= t.libraryid

The "Total" is a field in the T table, and isnull means that if it is not associated, use 0

MS SQL Update Set Select