offset用于指定从第几行开始取数据，在MySQL中，LIMIT 1, 2相当于LIMIT 2 OFFSET 1。
//src/Illuminate/Database/Query/Builder.php
/**
* Set the "offset" value of the query.
*
* @param int $value
* @return $this
*/
public function offset($value)
{
$property = $this->unions ? 'unionOffset' : 'offset';
$this->$property = max(0, $value);
return $this;
}//src/Illuminate/Database/Query/Grammers/Grammer.php
/**
* Compile the "offset" portions of the query.
*
* @param \Illuminate\Database\Query\Builder $query
* @param int $offset
* @return string
*/
protected function compileOffset(Builder $query, $offset)
{
return 'offset '.(int) $offset;
}