SQL Add values to rows from another table

Question

I'm having problems getting a SQL query to insert data into a column from another table. Whenever I do it it adds the date as extra rows rather than in the rows that match.

Here is an example:

TABLE A

ID	DATE	WEIGHTING
971	3/6/21
972	3/6/21
973	3/6/21

TABLE B

ID	DLTID	WEIGHTING
4441	971	.03
4443	972	.05
4445	973	.01

Join TABLE A on B.DTLID = A.ID

I need to populate the A.Weighting column with values from B.Weighting, matched on A.ID = B.DLTID. The tables have millions of rows. Hope this makes sense.

Thanks!

Answer 1

update TableA
set WEIGHTING = B.WEIGHTING
from tableB B
where B.DTLID = TableA.ID

and to improve the performance of this update, you can add an index on TableB (DTLID, WEIGHTING)