how to convert left join SQL script to Linq script? - sql

Question

How can I convert left join to linq script. I have a T-SQL like this:
SELECT
es.StandardID,
COUNT(DISTINCT esc.StandardCourseID) AS CourseIDCount,
COUNT(DISTINCT esp.StandardPostID) AS PostIDCount
FROM EduStandards AS es
LEFT JOIN EduStandardCourses AS esc
ON es.StandardID = esc.StandardID
LEFT JOIN EduStandardPosts AS esp
ON es.StandardID = esp.StandardID
GROUP BY es.StandardID
That I want to convert this to linq.

Answer 1

Following is the query with left join that is replica of your query in linq.
var query = (from es in dbContext.EduStandards
join esc in dbContext.EduStandardCourses on es.StandardID equals esc.StandardID into ssc
from esc in ssc.DefaultIfEmpty()
join esp in dbContext.EduStandardPosts on es.StandardID equals esp.StandardID into ssp
from esp in ssp.DefaultIfEmpty()
select new { StandardId = es.StandardID, CourseCount = ssc.Count(), PostCount = ssp.Count() }).Distinct().ToList();
But I think we need not to apply left join in linq to calculate count. Following optimized linq query will return same result.
var query2 = (from es in dbContext.EduStandards
join esc in dbContext.EduStandardCourses on es.StandardID equals esc.StandardID into ssc
join esp in dbContext.EduStandardPosts on es.StandardID equals esp.StandardID into ssp
select new { StandardId = es.StandardID, CourseCount = ssc.Count(), PostCount = ssp.Count() });

Answer 2

This is what I've come up with
var query = from es in db.EduStandards
join esc1 in db.EduStandardCourses
on es.StandardId equals esc1.StandardId into esc
from c in esc.DefaultIfEmpty()
join esp1 in db.EduStandardPosts
on es.StandardId equals esp1.StandardId into esp
from p in esp.DefaultIfEmpty()
group new { es.StandardId, Course = c, Post = p } by es.StandardId into g
select new
{
StandardId = g.Key,
CourseIdCount = g.Where(x => x.Course != null).Count(),
PostIdCount = g.Where(x => x.Post != null).Count(),
};
However, I'm not entirely sure if it'll work for EF.
You could always do something like this:
var query = from es in db.EduStandards
select new
{
es.StandardId,
CourseIdCount = db.EduStandardCourses.Where(esc => esc.StandardId == es.StandardId).Distinct().Count(),
PostIdCount = db.EduStandardPosts.Where(esp => esp.StandardId == es.StandardId).Distinct().Count()
};
Also, I can't attest to the performance of either one of these queries due to the lack of knowledge of your database.