Lambda表达式之查询篇
作者:互联网
Lambda表达式之查询篇
一般查询
1 db.User.Select(u => u); // 不带条件查询 2 3 db.User.Where(u => true); //不带条件查询 4 5 db.User.Where(u => u.username == "wjl" || u.username == "hyf"); // 带条件查询 || 表示 “或” && 表示 “且” 6 7 db.User.Select(u => u.username.EndsWith("丽")); // 模糊查询 相当于like '%丽' 8 9 db.User.Select(u => u.username.IndexOf("丽")); // 模糊查询 相当于like '%丽%' 10 11 db.User.Select(u => u.username.StartsWith("丽")); // 模糊查询 相当于like '丽%' 12 13 db.User.Where( u => (u.username == user.username && u.userpwd == user.userpwd)).Count(); // 计数 返回int类型的数值
聚合函数查询
//最大值 var list = from p in db.Products group p by p.CategoryID into g select new { g.Key, MaxPrice = g.Max(p => p.UnitPrice) }; //最小值 var q = from p in db.Products group p by p.CategoryID into g select new { g.Key, MaxPrice = g.Max(p => p.UnitPrice) }; //平均值 var q = from p in db.Products group p by p.CategoryID into g select new { g.Key, AveragePrice = g.Average(p => p.UnitPrice) }; //求和 var q = from p in db.Products group p by p.CategoryID into g select new { g.Key, TotalPrice = g.Sum(p => p.UnitPrice) }; //计数 var q = from p in db.Products group p by p.CategoryID into g select new { g.Key, NumProducts = g.Count() }; //带条件计数 var q = from p in db.Products group p by p.CategoryID into g select new { g.Key, NumProducts = g.Count(p => p.Discontinued) };
高级查询
//in查询 var list1 = db.Users.Where(u => new int[] { 1, 2, 3 }.Contains(u.Id)); var list2 = from u in db.Users where new int[] { 1, 2, 3 }.Contains(u.Id) select u; //分页查询,按需查询所要的字段 var list3 = db.Users.Where(u => new int[] { 1, 2, 3 }.Contains(u.Id)) .OrderBy(u => u.Id) .Select(u => new { Account = u.Account, Password = u.Password }).Skip(3).Take(5); var list4 = (from u in db.Users where new int[] { 1, 2, 3 }.Contains(u.Id) orderby u.Id select new { Account = u.Account, Pwd = u.Password }).Skip(3).Take(5); //多条件查询的另一种写法 var list5 = db.Users.Where(u => u.Name.StartsWith("小") && u.Name.EndsWith("新")) .Where(u => u.Name.EndsWith("新")) .Where(u => u.Name.Contains("小新")) .Where(u => u.Name.Length < 5) .OrderBy(u => u.Id); //连接查询,inner join var list6 = from u in db.Users join c in db.Companies on u.CompanyId equals c.Id where new int[] { 1, 2, 3, 4, 6, 7, 10 }.Contains(u.Id) select new { Account = u.Account, Pwd = u.Password, CompanyName = c.Name }; //连接查询,left join var list7 = from u in db.Users join c in db.Categories on u.CompanyId equals c.Id into ucList from uc in ucList.DefaultIfEmpty() where new int[] { 1, 2, 3, 4, 6, 7, 10 }.Contains(u.Id) select new { Account = u.Account, Pwd = u.Password };
分页查询,参数的动态改变自己去设置OrderBy为升序, OrderByDescending为降序 ,ThenByDescending与ThenBy为第二条件排序,Skip相当于not in ,Take相当于Top
1 var userlist = db.User.Where<User>(u => true).OrderByDescending(u => u.userid).ThenBy(u => u.username).Skip((pageindex - 1) * pagesize).Take(pagesize); 2 3 int pageindex; //从第几条开始 4 if (!int.TryParse(Request["pageindex"], out pageindex)) 5 { 6 pageindex = 1; 7 } 8 int rcordcount = db.User.Count(); //统计总记录数 9 int pagesize = 5; //每页要显示的记录条数 10 int pagecount = Convert.ToInt32(Math.Ceiling((double)rcordcount / pagesize)); //计算页数 11 12 pageindex = pageindex < 1 ? 1 : pageindex; //pageindex不能小于1 和 pageindex 不能大于记录总数 13 pageindex = pageindex > pagecount ? pagecount : pageindex; 14 15 // OrderBy为升序, OrderByDescending为降序 ,ThenByDescending与ThenBy为第二条件排序,Skip相当于not in ,Take相当于Top 16 var userlist = db.User.Where<User>(u => true).OrderByDescending(u => u.userid).ThenBy(u => u.username).Skip((pageindex - 1)* pagesize).Take(pagesize);
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标签:pageindex,int,db,查询,var,new,表达式,Lambda 来源: https://blog.csdn.net/fangyuan621/article/details/121293022