How to translate a complex SQL query with multiple joins, COUNT, and a left join into its LINQ equivalent?

Convert SQL containing multiple connections, count and left connection to linq

Given SQL query contains multiple connections (including left connections) and the aggregation of using the count and the count (distinct) function. The goal is to convert this query to its linq equivalent items for the data access layer of the application.

SQL query:

linq query understanding:

SELECT DISTINCT c.Id, 
       c.Title, 
       COUNT(v.Id) AS 'Nb_V2',
       COUNT(DISTINCT v.IdUser) AS 'Nb_V1',
       r.cnt AS 'Nb_R'
FROM TABLE_C c
JOIN TABLE_V v on c.Id = v.Id
LEFT JOIN ( 
    SELECT Id, COUNT(*)  AS cnt 
    FROM TABLE_R 
    GROUP BY Id
) r ON c.Id = r.Id
WHERE c.IdUser = '1234'
GROUP BY c.Id, c.Title, r.cnt

A method that converts the SQL query to the linq query understanding is to use nested connections and anonymous types. However, converting the count and count (differentinct) function to the linq syntax may be challenging. Lambda expression:

You can use LAMBDA expressions to implement a more concise solution for linq conversion. By using Join ... selectmany to operate and group, you can write the query as follows:

Translation Guide:

In order to effectively convert the SQL query to linq, the following guidelines can be applied:

var ans2 = Table_C.Where(c => c.IdUser == "1234")
                  .Join(Table_V, c => c.Id, v => v.Id, (c, v) => new { c, v })
                  .GroupJoin(Table_R.GroupBy(r => r.Id).Select(rg => new { Id = rg.Key, cnt = rg.Count() }), 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 }) //处理r可能为空的情况
                  .Select(cvrg => new { 
                      cvrg.Key.Title, 
                      Nb_V2 = cvrg.Count(), 
                      Nb_V1 = cvrg.Select(cvr => cvr.v.IdUser).Distinct().Count(), 
                      Nb_R = cvrg.Key.cnt 
                  });

Unless the child chooses to quote the external column, it should be converted independently.

A single single operator and polymerization operator can be directly applied to the results of the linq query.

Anonymous type is used to represent multiple columns.

• Connect classes are represented by navigation attributes or anonymous objects.
• The left connection needs to be used with GroupJoin ... selectmany. Pay attention to the situation where
may be empty, and the corresponding vacancy check is added to
• and
.
• This Revied Answer Improves The Linq Query by Handling The Potential Null Value of r In the GroupBy AND Select Clauses, ains this change. The Rest of the Explaanation Remains The Same , Providing A Clear and Accurate Translation Guide from SQL To Linq.

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