how can compare two columns in different table

Question

Hello friends, I have two table. 1. StudentAdmissiondetails column: AdmissionNumber and extra 2. StudentAcadmicdetails cloumn: AdmissionNumber and extra. I want to compare two AdmissionNumber column and if AdmisssionNumber in table 1 and not in table 2 mean I want to some task. this task i can do it. two tables may contain many sdmissionNumber. how can i do it.