FreeSql （二十四）Linq To Sql 語法使用介紹

時間 2019-11-05

標籤 freesql 二十四 linq sql 語法使用介紹欄目 SQL 简体版

原文原文鏈接

本來不支持 IQueryable 主要出於使用習慣的考慮，若是繼承 IQueryable，編寫代碼的智能總會提示出現一堆你不想使用的方法（對不起，我有強迫症），IQueryable 自身提供了一堆無法實現的方法，還有外部入侵的擴展方法，嚴重影響編碼體驗。以下圖：html

原覺得必須實現 IQueryable 才能夠實現，結果一次驚喜，原來只要有對應的方法就成。sql

雖然支持了，可是仍是推薦使用【鏈式 + lambda】！！！編碼

特別說明

此次功能更新，ISelect 增長了 5個方法，對【鏈式 + lambda】的用戶可能會形成少量影響，我在註釋上標明瞭，以下圖：code

特別是 .Select()，原先沒有支持，該功能與 ToList(a => new Dto{}) 合併實現的。htm

須要避免一下坑：blog

若是必定要使用 .Select() 方法，請務必在 .ToList() 以前調用它；繼承
請減小圖中方法在【鏈式 + labmda】模式下的使用；事務

全部 ISelect 均可以使用 linq to sql，包括 Repository、DbContext；get

Where

var t1 = (
    from a in fsql.Select<Student>()
    where a.id == item.id
    select a
).ToList();

Select(指定字段)

var t1 = (
    from a in fsql.Select<Student>()
    where a.id == item.id
    select new { a.id }
).ToList();

CaseWhen

var t1 = (
    from a in fsql.Select<Student>()
    where a.id == item.id
    select new {
        a.id,
        a.name,
        testsub = new {
            time = a.age > 10 ? "大於" : "小於或等於"
        }
    }
).ToList();

Join

var t1 = (
    from a in fsql.Select<Student>()
    join b in fsql.Select<School>() on a.id equals b.StudentId
    select a
).ToList();

var t2 = (
    from a in fsql.Select<Student>()
    join b in fsql.Select<School>() on a.id equals b.StudentId
    select new { a.id, bid = b.id }
).ToList();

var t3 = (
    from a in fsql.Select<Student>()
    join b in fsql.Select<School>() on a.id equals b.StudentId
    where a.id == item.id
    select new { a.id, bid = b.id }
).ToList();

LeftJoin

var t1 = (
    from a in fsql.Select<Student>()
    join b in fsql.Select<School>() on a.id equals b.StudentId into temp
    from tc in temp.DefaultIfEmpty()
    select a
).ToList();

var t2 = (
    from a in fsql.Select<Student>()
    join b in fsql.Select<School>() on a.id equals b.StudentId into temp
    from tc in temp.DefaultIfEmpty()
    select new { a.id, bid = tc.id }
).ToList();

var t3 = (
    from a in fsql.Select<Student>()
    join b in fsql.Select<School>() on a.id equals b.StudentId into temp
    from tc in temp.DefaultIfEmpty()
    where a.id == item.id
    select new { a.id, bid = tc.id }
).ToList();

From(多表查詢)

var t1 = (
    from a in fsql.Select<Student>()
    from b in fsql.Select<School>()
    where a.id == b.StudentId
    select a
).ToList();

var t2 = (
    from a in fsql.Select<Student>()
    from b in fsql.Select<School>()
    where a.id == b.StudentId
    select new { a.id, bid = b.id }
).ToList();

var t3 = (
    from a in fsql.Select<Student>()
    from b in fsql.Select<School>()
    where a.id == b.StudentId
    where a.id == item.id
    select new { a.id, bid = b.id }
).ToList();

GroupBy(分組)

var t1 = (
    from a in fsql.Select<Student>()
    where a.id == item.id
    group a by new {a.id, a.name } into g
    select new {
        g.Key.id, g.Key.name,
        cou = g.Count(),
        avg = g.Avg(g.Value.age),
        sum = g.Sum(g.Value.age),
        max = g.Max(g.Value.age),
        min = g.Min(g.Value.age)
    }
).ToList();