Problem with If statement with text across tables

I am coming over from Tabluea, and am stuck on an If Statement - I have a transaction table "Sheet1" with a unique customer id for each transaction that has a "market sector description" with one of four values: Muni, Mtge, Corp, or Govt. I have another customer table, linked to sheet1 by the same unique customer ID that has columns named Muni Cov, Mtge Cov, Corp Cov and Govt Cov, these columns each contain an alpha numeric code. What I am trying to do is this - if the transaction has a "market sector description" of 'Corp" I want to display the alpha numeric value from the 'Corp cov" column in the salesperson table, if it is a "Mtge" then pull the value from the "Mtge cov" column. In excel it would be easy.... here I am stuck. The same customer id can be on multiple transactions with different sector descriptions.