title: Aggregation function on Group by on multiple columns
body: <pre><code>WEEK    | CID | CUSTOMER_ID | L1
  2017-04 | 12  | 1           | ABC
  2017-04 | 13  | 1           | ABC
  2017-04 | 13  | 1           | ABC
  2017-04 | 16  | 1           | DFG
  2017-04 | 15  | 2           | DFG
  2017-14 | 14  | 1           | DFG
  </code></pre>
  
  <p>I have created a table by combining many databases (sample of the created table is above). I want the number of times each customer_id came in each week (such that like for <code>2017-04</code> week <code>customer_id 1</code> came in 3 times, we want to consider row 2 &amp; 3 as same (so 1 count for that) and then row 1 and row 4 makes it 3 times.</p>
  
  <p>So, output should be:</p>
  
  <pre><code>WEEK    | CUSTOMER_ID | COUNT(CUSTOMER_ID)
  2017-04 | 1           | 3
  2017-04 | 2           | 1
  2017-14 | 1           | 1
  </code></pre>
  
  <p>I'm trying something in the following lines:</p>
  
  <pre><code>select
      week, customer_id, count(customer_id)
  from
      table
  group by 
      week, cid, customer_id, L1;
  </code></pre>
  
  <p>But, I want to confirm doing like this will it count rows 2 &amp; 3 as one count or two counts?</p>