Compare and align rows of multiple but differing columns of data

I have a table of 4 columns. One column has county names, one
population and the other two have data relative to the county name for
the given year. Each year a new set of 4 columns is generated.
However, some but not all of the county names are in the subsequent
year. Same thing for each successive year. How can I:

1) Match the counties from year to year so they align in a row with
corresponding data? What happens if year being used for comparison
doesn't have county appearing in later year?
2) Make master list of counties (in left column) for all that have
appeared in the data sets?
3) Perform this operation on multiple years worth of data at a time.

Typically I need to create a table that has a list in the left column
of all the county names that have appeared in any of the subject years
(year one might have 25 names, year two 30 but some will obviously be
different and not all of year one will be included, year three 22 etc).
I then need to extract and list next to the county names, a specific
column of corresponding data (in this case the population of the
county)

I can send file of raw data and the sorted finished table if my example
explained above does not clearly explain the problem I am trying to
solve.

Thank you