SQL to LINQ: Transforming a Grouped Query with Left Join

This article provides a step-by-step guide to converting a SQL query into its equivalent LINQ expression. The example focuses on a query that performs a left join, grouping, and ordering. Let's break down the SQL query and its LINQ translation.

Original SQL Query:

SELECT  A.StationNo,COUNT(A.StationNo)
FROM [Mesdatabase_6631].[dbo].[Work_Process] A
LEFT JOIN Base_Station B ON A.StationNo=B.StationNo

GROUP BY 
A.StationNo,B.SortIndex
ORDER BY   B.SortIndex

Equivalent LINQ Expression:

var result = from A in db.Work_Process
             join B in db.Base_Station on A.StationNo equals B.StationNo into AB
             from B in AB.DefaultIfEmpty()
             group A by new { A.StationNo, B.SortIndex } into g
             orderby g.Key.SortIndex
             select new { StationNo = g.Key.StationNo, Count = g.Count() };

Explanation:

1. from A in db.Work_Process: This line represents the FROM clause in SQL, specifying that we are selecting data from the Work_Process table and assigning it to the variable A.
2. join B in db.Base_Station on A.StationNo equals B.StationNo into AB: This implements the left join. It joins the Work_Process table (aliased as A) with the Base_Station table (aliased as B) based on the StationNo column. The result of the join is stored in AB.
3. from B in AB.DefaultIfEmpty(): This handles the left join aspect. It retrieves all records from the Work_Process table, even if there is no matching record in the Base_Station table. If a match is not found, a default value (null in this case) is returned.
4. group A by new { A.StationNo, B.SortIndex } into g: This represents the GROUP BY clause, grouping the data by StationNo and SortIndex (columns from the joined tables). The grouped data is assigned to the variable g.
5. orderby g.Key.SortIndex: This is the ORDER BY clause, sorting the grouped data based on the SortIndex column. Note that g.Key refers to the object containing the group key values.
6. select new { StationNo = g.Key.StationNo, Count = g.Count() }: This line specifies the data to be selected. For each group (g), it selects the StationNo and the count of elements in the group (g.Count()).

Key Points:

• The LINQ expression uses the into keyword to create a temporary variable (AB) to store the intermediate join results, enabling us to process the left join in a more concise way.
• The DefaultIfEmpty() method is used to include records from the Work_Process table that do not have a match in the Base_Station table, maintaining the left join behavior.
• The group by clause is used to group the data by the desired columns, and the orderby clause sorts the groups based on the SortIndex column.

By understanding the conversion process and the corresponding elements in both SQL and LINQ, you can effectively translate SQL queries into LINQ expressions and leverage the powerful features of LINQ for querying and data manipulation.