LinqToSql

原本不支持 IQueryable 主要出于使用习惯的考虑，如果继承 IQueryable，编写代码的智能总会提示出现一堆你不想使用的方法（对不起，我有强迫症），IQueryable 自身提供了一堆没法实现的方法，还有外部入侵的扩展方法，严重影响编码体验。如下图：

原以为必须实现 IQueryable 才可以实现，结果一次惊喜，原来只要有对应的方法就成。

虽然支持了，但是还是推荐使用【链式 + lambda】 ！！！

特别说明

需要避免一下坑：

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

• 请减少图中方法在【链式 + labmda】模式下的使用；

v1.4.0+ 版本请使用以下命令安装（老版本不需要安装）：

dotnet add package FreeSql.Extensions.LinqSql

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();

参考资料

• 《多表查询》
• 《返回数据》
• 《仓储层Repository》
• 《优化之：延时加载》
• 《优化之：贪婪加载》
• 《Expression 表达式函数》