title: Calculate Weighted Average in sql for duplicate items
body: <p><strong>TABLE</strong> </p>
  
  <pre><code>ITEM |  SKU  |  QUANT  |      ITEM LANDING
  -----+-------+---------+-----------------
  ABC  | A-B-C |     5   |         500  
  ABC  | A-B-C |    10   |         400  
  DEF  | D-E-F |     5   |         200  
  GHI  | G-H-I |     6   |         300  
  JKL  | J-K-L |     5   |         800  
  JKL  | J-K-L |    10   |         600 
  </code></pre>
  
  <p><strong>RESULT</strong></p>
  
  <pre><code>ITEM NAME | ITEM SKU  | QUANT  |  Weighted Average
  ----------+-----------+--------+---------------------
    ABC     |    A-B-C  |   15   |      433.33  
    DEF     |    D-E-F  |    5   |      200  
    GHI     |    G-H-I  |    6   |      300  
    JKL     |    J-K-L  |   15   |      666.66  
  </code></pre>
  
  <p>I want to calculate weighted average for duplicate items<br>
  <strong>((first landing *qty)+(second landing *qty))/(sum(qty)</strong></p>