Lambda表達式之查詢篇

一般查詢

db.User.Select(u => u); // 不帶條件查詢

db.User.Where(u => true); //不帶條件查詢

db.User.Where(u => u.username == "wjl" || u.username == "hyf"); // 帶條件查詢  || 表示 “或” && 表示 “且”

db.User.Select(u => u.username.EndsWith("麗")); // 模糊查詢 相當於like '%麗' 

db.User.Select(u => u.username.IndexOf("麗")); // 模糊查詢 相當於like '%麗%' 

db.User.Select(u => u.username.StartsWith("麗")); // 模糊查詢 相當於like '麗%' 

db.User.Where( u => (u.username == user.username && u.userpwd == user.userpwd)).Count(); // 計數 返回int類型的數值

聚合函數查詢

//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)
        };
//計數
var q = from p in db.Products
        group p by p.CategoryID into g
        select new
        {
            g.Key,
            NumProducts = g.Count()
        };
//帶條件計數
var q = from p in db.Products
        group p by p.CategoryID into g
        select new
        {
            g.Key,
            NumProducts = g.Count(p => p.Discontinued)
        };

高級查詢

//in查詢
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;

//分頁查詢,按需查詢所要的字段
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);

//多條件查詢的另一種寫法
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);

//連接查詢，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
            };
//連接查詢，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
            };

分頁查詢，參數的動態改變自己去設置OrderBy為升序， OrderByDescending為降序 ，ThenByDescending與ThenBy為第二條件排序，Skip相當於not in ，Take相當於Top

var userlist = db.User.Where<User>(u => true).OrderByDescending(u => u.userid).ThenBy(u => u.username).Skip((pageindex - 1) * pagesize).Take(pagesize);

int pageindex; //從第幾條開始
if (!int.TryParse(Request["pageindex"], out pageindex))
{
pageindex = 1;
}
int rcordcount = db.User.Count(); //統計總記錄數
int pagesize = 5; //每頁要顯示的記錄條數
int pagecount = Convert.ToInt32(Math.Ceiling((double)rcordcount / pagesize)); //計算頁數

pageindex = pageindex < 1 ? 1 : pageindex; //pageindex不能小於1 和 pageindex 不能大於記錄總數
pageindex = pageindex > pagecount ? pagecount : pageindex;

// OrderBy為升序， OrderByDescending為降序 ，ThenByDescending與ThenBy為第二條件排序，Skip相當於not in ，Take相當於Top
var userlist = db.User.Where<User>(u => true).OrderByDescending(u => u.userid).ThenBy(u => u.username).Skip((pageindex - 1)* pagesize).Take(pagesize);