title: Average Population of Each Continent
body: <p>am trying to write the query for the following requirement.Given two tables, City and Country whose description are given below. Print the name of all continents (key: Country.Continent) along with the average City population rounded down to nearest integer.</p>
  
  <pre><code>City
  
  +-------------+----------+
  | Field       | Type     |
   +-------------+----------+
  | ID          | int(11)  |
  | Name        | char(35) |
  | CountryCode | char(3)  |
  | District    | char(20) |
  | Population  | int(11)  |
  +-------------+----------+
  </code></pre>
  
  <p>Country</p>
  
  <pre><code>+----------------+-------------+
  | Field          | Type        |
  +----------------+-------------+
  | Code           | char(3)     |
  | Name           | char(52)    |
  | Continent      | char(50)    |
  | Region         | char(26)    |
  | SurfaceArea    | float(10,2) |
  | IndepYear      | smallint(6) |
  | Population     | int(11)     |
  | LifeExpectancy | float(3,1)  |
  | GNP            | float(10,2) |
  | GNPOld         | float(10,2) |
  | LocalName      | char(45)    |
  | GovernmentForm | char(45)    |
  | HeadOfState    | char(60)    |
  | Capital        | int(11)     |
  | Code2          | char(2)     |
  +----------------+-------------+
  </code></pre>
  
  <p>PS #1: City.CountryCode and Country.Code is same key.
  PS #2: Continent without cities should not be included in output.</p>
  
  <p>I have tried the following query, but it seems something wrong in it,
  please correct me.</p>
  
  <pre><code>select Country.Continent,round(avg(City.population),0) from City
  join Country
  on City.CountryCode=Country.Code
  where City.ID&lt;&gt;NULL
  Group by Country.Continent;
  </code></pre>