C# Linq to MySQL query with join makes bad SQL?

Question

I'm using C# to write LINQ in to a MySQL database. I think the SQL generated might be wrong for a simple table join that I'm doing.

My nuget packages are Mysql.Data v6.9.9, Mysql.data.entities v6.8.3, and MySql.data.entity v6.9.9

The LINQ is this:

query = from peopleResult in query
    join t in technologyQuery on peopleResult.Company_Id equals t.Company_Id
    select peopleResult;

The SQL generated looks like this:

SELECT ...
FROM `people` AS `Extent1` 
INNER JOIN `technologies` AS `Extent2` ON (`Extent1`.`Company_Id` = `Extent2`.`Company_Id`) OR ((`Extent1`.`Company_Id` IS  NULL) AND (`Extent2`.`Company_Id` IS  NULL))
WHERE ...

Is this part of the join right?

(`Extent1`.`Company_Id` IS  NULL) AND (`Extent2`.`Company_Id` IS  NULL)

The query is incredibly long running when that is included. I pulled that out of the SQL with a regex, and it runs much faster and seems to give the correct results.

Is my LINQ incorrect or missing something? Does the MySQL linq-to-sql likely have a bug?

Thank you for your time thinking about this.

Answer 1

It's not a MySQL connector bug, but EF feature which tries to emulate the C# equality rules for nullable types.

First, make sure to set DbContext.Configuration.UseDatabaseNullSemantics to true , for instance inside your DbContext derived class constructor:

Configuration.UseDatabaseNullSemantics = true;

By idea this should solve the issue. However they implemented it for comparison operators and forgot the joins. So you have to use the alternative join syntax with where clause:

query = 
    from peopleResult in query
    from t in technologyQuery
    where peopleResult.Company_Id == t.Company_Id
    select peopleResult;

which will be translated to the desired SQL JOIN without IS NULL part.