查询

1.查询所有的。

var query = from p in _Context.Info select p;

var query = _Context.Info;

2.单条件查询

等值查

var query = from p in _Context.Info where p.Code == "p002" select p;

注意：双等号，双引号。---C#语法。

var query = _Context.Info.Where(p => p.Code == "p002" );  //Lambda表达式

不等值查

            //var query = from p in _Context.Info where p.Code != "p002" select p;

            //var query = _Context.Info.Where(p => p.Code != "p002");  //Lambda表达式

            //var query = from p in _Context.Info where p.Birthday.Value.Year > 1985 select p;

            var query = _Context.Info.Where(p=>p.Birthday.Value.Year < 1985);

3.多条件查询

            //var query = from p in _Context.Info where p.Sex == true && p.Birthday.Value.Year > 1985 select p;

            //var query = _Context.Info.Where(p=>p.Sex==true && p.Birthday.Value.Year>1985);

            var query = _Context.Info.Where(p => p.Sex == true).Where(p=>p.Birthday.Value.Year>1985) ; //多条件逻辑与可以用链式表达式写         

4.模糊查询

 

            Contains 确认查询字是否包含

StartsWith 确认查询字开头是否与字符串匹配

EndsWith 确认查询字结尾是否与字符串匹配

Substring 查询字符串从指定字符开始具有指定的长度

            //var query = from p in _Context.Info where p.Name.Contains("张") select p;  //相当于like '%张%'

            //var query = from p in _Context.Info where p.Name.StartsWith("张") select p; // like '张%'

            //var query = from p in _Context.Info where p.Name.EndsWith("张") select p;  //like '%张'

            //var query = from p in _Context.Info where p.Name.Substring(1,1) == "张" select p;// like '_张%'

 

            var query = _Context.Info.Where(p => p.Name.StartsWith("张"));

 

5.集合操作

var q1 = from p in _Context.Info where p.Sex== true select p;

var q2 = _Context.Info.Where(p => p.Birthday.Value.Year > 1985);

            

交

          Intersect      对值进行比较生成2个序列的交集

var query = q1.Intersect(q2);

并

            Union  生成2个序列的并集

var query = q1.Union(q2);

差

               Except  对值进行比较产生2个序列的差集

var query = q2.Except(q1);

 

6.统计函数：

            var query = _Context.Info.Where(p => p.Birthday.Value.Year > 1985);

            //this.Text = query.Count().ToString();

            //this.Text = query.Sum(p=>DateTime.Now.Year - p.Birthday.Value.Year).ToString();

            //this.Text = query.Average(p => DateTime.Now.Year - p.Birthday.Value.Year).ToString();

            //this.Text = query.Max(p => DateTime.Now.Year - p.Birthday.Value.Year).ToString();

            this.Text = query.Min(p => DateTime.Now.Year - p.Birthday.Value.Year).ToString();

7.转换操作

   Tolist()  创建一个集合

   ToArray()  创建一个数组

    First()    创建一个对象  first第一个

ToList(),ToArray(),First()/Single()

8.分页

              kip(2*2) 跳过2条 分成2也  Take(2)显示2条

var query = _Context.Info.Skip(2*2).Take(2);