Linq to Sql

nicyeNovember 21, 2021About 2 minAbout 577 words

Linq to Sql

Originally not supporting IQueryable is mainly due to the consideration of usage habits. The intelligence of writing code will always prompt a bunch of methods you don't want to use. IQueryable itself provides a bunch of methods that cannot be implemented, as well as external intrusion extension methods, which seriously affect Coding experience. As shown below:

image

For FreeSql v1.4.0+ version, please use the following items to install with commands (old version does not need to be installed):

dotnet add package FreeSql.Extensions.Linq

Special Note

  • Please try not to use the following Linq methods in ISelect mode: GroupJoin, Select, SelectMany, Join and DefaultIfEmpty.

  • If you must use the .Select() method in ISelect, be sure to call it before .ToList().

IQueryable

FreeSql provides a powerful data query object: ISelect.

FreeSql.Extensions.Linq v1.4.0+ implements the common functions of IQueryable query objects for interactive use in various frameworks.

//Convert ISelect to IQueryable
IQueryable<Student> queryable = fsql.Select<Student>().AsQueryable();

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

//Restore IQueryable to ISelect
ISelect<Studeng> select = queryable.RestoreToSelect();

Note: The implementation of IQueryable does not currently support GroupBy. You can consider using the RestoreSelect method to switch back to ISelect for query.

Where

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

Specify Fields: Select

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();

Multi-table Query: From

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();

Grouping: GroupBy

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();

Reference