title: Groupby with sum on Julia Dataframe
body: <p>I am trying to make a groupby + sum on a Julia Dataframe with Int and String values</p>
  <p>For instance, df :</p>
  <pre><code>│ Row │ A      │ B      │ C     │ D      │
  │     │ String │ String │ Int64 │ String │
  ├─────┼────────┼────────┼───────┼────────┤
  │ 1   │ x1     │ a      │ 12    │ green  │
  │ 2   │ x2     │ a      │ 7     │ blue   │
  │ 3   │ x1     │ b      │ 5     │ red    │
  │ 4   │ x2     │ a      │ 4     │ blue   │
  │ 5   │ x1     │ b      │ 9     │ yellow │
  </code></pre>
  <p>To do this in Python, the command could be :</p>
  <pre><code>df_group = df.groupby(['A', 'B']).sum().reset_index()
  </code></pre>
  <p>I will obtain the following output result with the initial column labels :</p>
  <pre><code>    A  B   C
  0  x1  a  12
  1  x1  b  14
  2  x2  a  11
  </code></pre>
  <p>I would like to do the same thing in Julia. I tried this way, unsuccessfully :</p>
  <pre><code>df_group = aggregate(df, [&quot;A&quot;, &quot;B&quot;], sum)
  </code></pre>
  <p><em>MethodError: no method matching +(::String, ::String)</em></p>
  <p>Have you any idea of a way to do this in Julia ?</p>