How to update a table based on values in another table?

[SHDJason]

I have 2 tables. I need to update some of the values in table A based on values in table B.  This is part of a bigger function.

Table A has two columns. Source and Tactic.

Table B has one colum. Source

If Table A source = one of the values in Table B, then Tactic should be X else Y.

How to do this? Do I need to iterate through all the values in Table B each time? There are many thousands of rows. 

Note: Tables A and B are updated independantly.

Thanks for your help.

[Greg_Deckler · Accepted solution]

Take a look at my article here, it should address your issue.

http://community.powerbi.com/t5/Community-Blog/Good-Ol-VLOOKUP-The-Ultimate-Guide-to-Lookups-in-Powe...