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copied!<p>By using the <code>merge</code> function and its optional parameters:</p> <p><strong><em>Inner join:</em></strong> <code>merge(df1, df2)</code> will work for these examples because R automatically joins the frames by common variable names, but you would most likely want to specify <code>merge(df1, df2, by = "CustomerId")</code> to make sure that you were matching on only the fields you desired. You can also use the <code>by.x</code> and <code>by.y</code> parameters if the matching variables have different names in the different data frames.</p> <p><strong><em>Outer join:</em></strong> <code>merge(x = df1, y = df2, by = "CustomerId", all = TRUE)</code></p> <p><strong><em>Left outer:</em></strong> <code>merge(x = df1, y = df2, by = "CustomerId", all.x = TRUE)</code></p> <p><strong><em>Right outer:</em></strong> <code>merge(x = df1, y = df2, by = "CustomerId", all.y = TRUE)</code></p> <p><strong><em>Cross join:</em></strong> <code>merge(x = df1, y = df2, by = NULL)</code></p> <p><strike>Just as with the inner join, you would probably want to explicitly pass "CustomerId" to R as the matching variable.</strike> I think it's almost always best to explicitly state the identifiers on which you want to merge; it's safer if the input data.frames change unexpectedly and easier to read later on.</p> <p>You can merge on multiple columns by giving <code>by</code> a vector, e.g., <code>by = c("CustomerId", "OrderId")</code>. </p> <p>If the column names to merge on are not the same, you can specify, e.g., <code>by.x = "CustomerId_in_df1",</code>by.y = "CustomerId_in_df2"<code>where</code>CustomerId_in_df1<code>is the name of the column in the first data frame and</code>CustomerId_in_df2` is the name of the column in the second data frame. (These can also be vectors if you need to merge on multiple columns.)</p>

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