LinqToSql

linq to sql 写法过于生硬不灵活，left join 写法非常不好，建议慢慢放充转向 Labmda 链式风格，早日走向康庄大道。

IQueryable 方法入侵问题，如下图很多不能实现的，以及第三方包入侵的扩展方法，严重影响编程体验。

image

dotnet add package FreeSql.Extensions.Linq

特别说明

• 请尽量不要在 ISelect 模式下的使用 Linq 方法：GroupJoin、Select、SelectMany、Join、DefaultIfEmpty；

• 如果一定要在 ISelect 中使用 .Select() 方法，请务必在 .ToList() 之前调用它；

IQueryable

FreeSql 提供强大的数据查询对象 ISelect。

FreeSql.Extensions.Linq v1.4.0+ 实现了 IQueryable 查询对象常用功能，以便在各框架中交互使用。

//将 ISelect 转为 IQueryable
IQueryable<Student> queryable = fsql.Select<Student>().AsQueryable();

//Linq 方法查询
var t1 = queryable.Where(a => a.id == 1).FirstOrDefault();

//将 IQueryable 还原为 ISelect
ISelect<Studeng> select = queryable.RestoreToSelect();

注意：IQueryable 的实现目前不支持 GroupBy，可以考虑使用 RestoreSelect 方法转回 ISelect 进行查询

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 ? "大于" : "小于或等于"
        }
    }
).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(多表查询)

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(分组)

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();