>[danger] where 条件设计稍微有点复杂，所以单独拿出来
>多数情况下使用数组参数即可满足要求
>在实现复杂的where条件时，如果数组参数满足不了要求，可以使用字符串参数
>字符串参数 框架将不做解析，直接交给PDO处理
>在书写字符串参数时应尽量遵循SQL的书写规范
## 基本操作
$where['id'] = 10; // where `id`=10; 等于
$where['id >'] = 10; // where `id`>10; 大于
$where['id <'] = 10; //where `id`<10; 小于
$where['id >='] = 10; //where `id`>=10; 大于等于
$where['id <='] = 10; //where `id`<=10; 小于等于
$where['id <>'] = 10; //where `id`<>10; 不等于
**IN 和 NOT IN**
$where['id'] = [2,4,6,8]; //where `id` IN(2,4,6,8);
$where['id'] = ['IN',[2,4,6,8]]; //同上 指定操作符是'IN'
$where['id'] = ['NOT IN',[2,4,6,8]]; //where `id` NOT IN(2,4,6,8)
**BETWEEN 和 NOT BETWEEN**
$where['id'] = ['BETWEEN',[1,10]] //WHERE `id` BETWEEN 1 AND 10
$where['id'] = ['NOT BETWEEN',[1,10]] //WHERE `id` NOT BETWEEN 1 AND 10
**LIKE**
$where['name'] = ['LIKE','tom'] //WHERE `name` LIKE %tom%
$where['name'] = '%tom%' //WHERE `name` LIKE %tom%
**多个字段对应同一个值的情况：**
$where['id|uid'] = 10; //where `id`=10 OR `uid`=10
## 连接多个条件
**默认使用 AND 连接多个条件**
$where['id >'] = 10;
$where['pot >'] = 100;
//...更多
where `id`>10 AND `pot`>100 AND ...更多 //合并后的条件
**OR**
$where['id >'] = 10;
$where['pid <'] = ['OR',20];
WHERE `id`>10 OR `pid`<20 //合并后的语句
也可以这么写：
$where['OR pid <'] = 20;
也可以写在一个数组里面
$where = ['id >'=>10,'OR pid <'=>20];
>[danger]where条件可以多次调用以应对稍微复杂一点的条件
$m = D('user');
$where1 = ['id >'=>10,'OR pot >'=>100];
$where2 = ['OR pid <'=> 20];
$user = $m->where($where1)->where($where2)->select();
//两次调用where()函数合并之后的 where 条件：
WHERE (`id `>10 OR `pot `>100) OR (`pid`<20)
## 使用字符串参数
**框架不做解析，也不绑定参数，需注意语句安全**
$where = "`id`>10 AND `pot`>100";
**绑定参数的方式：**
$m = D('user');
$where = "`id`>:id AND `pot`>:pot"; //将参数绑定到 :id 和 :pot 上
$arr = [':id'=>10,':pot'=>100]; //给绑定参数赋值
$user = $m->where($where,$arr)->select(); //$arr作为第二个参数传入where函数
