Macro for matching random results in one spreadsheet with another..*gulp*

[Accipiter]

ok, So I have two workbooks; Master, and Union. Master is a list of everyone my company insures basically, and Union contains just the union people my company insures. Now, what I need this macro to do is search the Union workbook for a social security number (in column A). Then I need it to look on a worksheet (named Eligible) within Master, and if the same social security number exists on this worksheet. If it does, I need it to place a Y in column AC of the row that the social security number appears in. Otherwise, if it appears in Master and not in Union, I need an 'N' to be displayed at that point. In other words, let's say social security number '123456789' appears in Union, and also Master, I need a 'Y' to appear in row 2, column AC of Master. And say social security number 987654321 appears in Master, but not Union, I need an N to be displayed in row 3, column AC. I'm completely stumped as to where to even start on this one....

_ A__________AC
1 SSN________Match
2 123456789__Y
3. 987654321__N

[Accipiter]

hmm no ideas anyone?


edit....wait did i just describe a vlookup....