Depending on the results you are looking for, choosing whether to do a "matching A with all B" relationship (FULL OUTER JOIN in SQL) or Append Data Extensions can sometimes be a bit tricky.
In general, when trying to merge data from multiple Source Data Extension (input sources) into one Target Data Extension (output source), Append Data Extensions is the way to go.
To see the difference in the results, let's look at an example scenario starting with two Data Extensions that contain customer and lead information.
Match All Customers with All Leads (FULL OUTER JOIN in SQL)
This will create rows with data from both Data Extensions (DEs) based on one or more matching columns. In this example, the DEs will match on Id. Here's what the results will look like after doing a Match All Customers with All Leads (Full Outer Join).
APPEND DATA EXTENSIONS
This action will bring all the rows of each Data Extension into one new Data Extension. Here's what the results will look like after doing an Append DEs.