Python的Django框架中的数据过滤功能
我们很少会一次性从数据库中取出所有的数据；通常都只针对一部分数据进行操作。 在Django API中，我们可以使用`` filter()`` 方法对数据进行过滤：
>>> Publisher.objects.filter(name='Apress') [<Publisher: Apress>]
filter() 根据关键字参数来转换成 WHERE SQL语句。 前面这个例子 相当于这样：
SELECT id, name, address, city, state_province, country, website FROM books_publisher WHERE name = 'Apress';
你可以传递多个参数到 filter() 来缩小选取范围：
>>> Publisher.objects.filter(country="U.S.A.", state_province="CA") [<Publisher: Apress>]
多个参数会被转换成 AND SQL从句， 因此上面的代码可以转化成这样：
SELECT id, name, address, city, state_province, country, website FROM books_publisher WHERE country = 'U.S.A.' AND state_province = 'CA';
注意，SQL缺省的 = 操作符是精确匹配的， 其他类型的查找也可以使用：
>>> Publisher.objects.filter(name__contains="press") [<Publisher: Apress>]
在 name 和 contains 之间有双下划线。和Python一样，Django也使用双下划线来表明会进行一些魔术般的操作。这里，contains部分会被Django翻译成LIKE语句：
SELECT id, name, address, city, state_province, country, website FROM books_publisher WHERE name LIKE '%press%';
其他的一些查找类型有：icontains(大小写无关的LIKE),startswith和endswith, 还有range(SQLBETWEEN查询）。
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