Use factory methods to create SQL statement instead of using raw string embedded in template code

For example:

``` ruby
select(:all).from(:articles)
-> SELECT * FROM articles;
```

``` ruby
select(:all).from(:comments).where(private_id: "\(private_id)")
-> SELECT * FROM comments WHERE private_id = "\(private_id)";
```

``` ruby
select(:all).from(:comments).where(private_id: select
```
