http://drupal.org/developing/api/database



SELECT QUERY

  //SELECT column1, column2, columnN 
  //FROM {tablename} 
  //WHERE column1 = $value1 
  //AND column2 <> $value2
  //ORDERBY 'sticky', DESC
  //ORDERBY 'created', DESC
  //LIMIT 10;

  $query = db_select('tablename', 'optional alias')
    ->fields('tablename or alias if given', array('colunm1', 'column2' ..., 'columnN'))
    ->condition('column1', 'value1')
    ->condition('column2', 'value2', 'optional <>')
    ->orderBy('sticky', 'DESC')
    ->orderBy('created', 'DESC')
    ->limit(variable_get('default_nodes_main', 10))
    ->extend('PagerDefault');
  return $query->execute()->fetch();
  //Calling fields() with no field list will result in a "SELECT *" query.



INSERT QUERY

  //INSERT INTO {tablename} ('column1', 'column2') VALUES ('value1', 'value2');

  $query = db_insert('tablename')
    ->fields(array('column1', 'column2'))
    ->values(array('column1' => 'value1', 'column2' => 'value2'))
    ->execute();



UPDATE QUERY

  //UPDATE {tablename}
  //SET column1 = value1, column2 = value2
  //WHERE column3 >= value3
  
  $query = db_update('tablename')
    ->fields(array('column1' => 'value1', 'column2' => 'value2'))
    ->condition('column3', 'value3', '>=')
    ->execute();
  //The third parameter for ->condition() is optional and defaults to "="



MERGE QUERY

  //In tablename, if the field = value exists, update field1 = value1 && field2 = value2 
  //UPDATE {tablename} SET column1 = value1, column2 = value2 WHERE column = value

  //If tablename, if the field = value does NOT exists, 
  //INSERT INTO {tablename} ('column', 'column1', 'column2') VALUES ('value', 'value1', 'value2');

  $query = db_merge('tablename')
    ->key(array('column' => 'value'))
    ->fields(array(
        'column1' => 'value1',
        'column2' => 'value2',
    ))
    ->execute();



DELETE QUERY
  //DELETE FROM {tablename} WHERE column = value;
  $query = db_delete('tablename')
    ->condition('column', 'value')
    ->execute();