How to Translate Complex SQL Queries with Multiple Joins, Counts, and Left Joins into LINQ?

Convert complex multi-table join, count and left join SQL queries to LINQ

When your existing SQL queries involve complex joins, counts, and left joins, you need to convert them into equivalent LINQ expressions. Let’s break down the process and address the specific challenges encountered.

Understand the conversion rules

Converting SQL to LINQ requires understanding the specific conversion rules involved. Some key principles include:

• Separate subqueries can be converted into separate variables, while subqueries referencing external columns require parentheses.
• Join clauses can be expressed by combining table aliases and equality conditions.
• Left joins are implemented using navigation properties and the DefaultIfEmpty() method.
• Aggregation functions and counts can be implemented using LINQ aggregates (e.g., Count(), Distinct().Count()).

Conversion of SQL queries

Considering the provided SQL query, we first define the subquery for calculating the count:

<code class="language-csharp">var subrq = from r in Table_R
            group r by r.Id into rg
            select new { Id = rg.Key, cnt = rg.Count() };</code>

Now, for the main query:

<code class="language-csharp">var ansq = (from c in Table_C
            join v in Table_V on c.Id equals v.Id
            join r in subrq on c.Id equals r.Id into rj
            from r in rj.DefaultIfEmpty()
            where c.IdUser == "1234"
            group new { c, v, r } by new { c.Id, c.Title, r.cnt } into cvrg
            select new {
                cvrg.Key.Title,
                Nb_V2 = cvrg.Count(),
                Nb_V1 = cvrg.Select(cvr => cvr.v.IdUser).Distinct().Count(),
                Nb_R = (int?)cvrg.Key.cnt
            }).Distinct();</code>

This LINQ expression performs the necessary join, group and count operations.

Conversion of Lambda expressions

For the conversion of lambda expressions, we can use the GroupJoin() and SelectMany() methods to handle left joins:

<code class="language-csharp">var subr2 = Table_R.GroupBy(r => r.Id).Select(rg => new { Id = rg.Key, cnt = rg.Count() });
var ans2 = Table_C.Where(c => c.IdUser == "1234")
                  .Join(Table_V, c => c.Id, v => v.Id, (c, v) => new { c, v })
                  .GroupJoin(subr, cv => cv.c.Id, r => r.Id, (cv, rj) => new { cv.c, cv.v, rj })
                  .SelectMany(cvrj => cvrj.rj.DefaultIfEmpty(), (cvrj, r) => new { cvrj.c, cvrj.v, r })
                  .GroupBy(cvr => new { cvr.c.Id, cvr.c.Title, cvr.r.cnt })
                  .Select(cvrg => new { cvrg.Key.Title, Nb_V2 = cvrg.Count(), Nb_V1 = cvrg.Select(cvr => cvr.v.IdUser).Distinct().Count(), Nb_R = (int?)cvrg.Key.cnt });</code>

This lambda-style expression achieves the same task as query comprehension.

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