Implement sum and average aggregation for decimal in SQLite

[ranma42]

Contributes to #19635

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        Summary of the changes
        - Detail 1
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        Fixes #bugnumber

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[ranma42]

Some notes:

• I ended up duplicating CombineTerms

  efcore/src/EFCore.Relational/Query/Internal/QueryableAggregateMethodTranslator.cs

  Lines 160 to 180 in 403ea80

  	private SqlExpression CombineTerms(EnumerableExpression enumerableExpression, SqlExpression sqlExpression)
  	{
  	if (enumerableExpression.Predicate != null)
  	{
  	if (sqlExpression is SqlFragmentExpression)
  	{
  	sqlExpression = _sqlExpressionFactory.Constant(1);
  	}
  	sqlExpression = _sqlExpressionFactory.Case(
  	new List<CaseWhenClause> { new(enumerableExpression.Predicate, sqlExpression) },
  	elseResult: null);
  	}
  	if (enumerableExpression.IsDistinct)
  	{
  	sqlExpression = new DistinctExpression(sqlExpression);
  	}
  	return sqlExpression;
  	}

  It looks like most IAggregateMethodCallTranslator implementations are re-implementing this same operation, so it might be ok (even intentional to keep them independent), but I was not expecting it at first
• I am not 100% confident about the handling of nullability:
  • in LINQ .Sum over an empty decimal enumerable returns a decimal 0
  • in LINQ .Sum over an empty decimal? enumerable returns a decimal? null
  • in LINQ .Sum over an empty decimal? enumerable returns a decimal? 0
  • in LINQ .Sum over a decimal? with some null would return null (I believe)
  • in LINQ .Sum over a decimal? with some nulls would ignore all of them
    being unsure, I replicated the handling of SUM

    efcore/src/EFCore.Relational/Query/SqlNullabilityProcessor.cs

    Lines 1394 to 1403 in 403ea80

    	if (sqlFunctionExpression.IsBuiltIn
    	&& string.Equals(sqlFunctionExpression.Name, "SUM", StringComparison.OrdinalIgnoreCase))
    	{
    	nullable = false;
    	return _sqlExpressionFactory.Coalesce(
    	sqlFunctionExpression.Update(instance, arguments),
    	_sqlExpressionFactory.Constant(0, sqlFunctionExpression.TypeMapping),
    	sqlFunctionExpression.TypeMapping);
    	}

• adding/removing the CombineTerms step does not affect any of the existing tests; I plan to add at least a test that checks for that, but I might need some guidance in what is the best place to do so (should it be sqlite-specific or generic?)
• adding/removing the nullability processing changes the SQL output, hence its effect is tested. OTOH, the results of the evaluation seem to pass regardless of that change, which might point out that additional tests are needed for that as well

EDIT: corrected notes on LINQ .Sum() after testing

[ranma42]

combined sum+avg as suggested in #33720 (comment)
the max/min aggregation is left for a future PR as it might involve additional design around how to handle other types (ulong, TimeSpan, etc)

[cincuranet]

@ranma42 Can you please resolve the conflict and then I can merge. Thanks.

[ranma42]

@cincuranet rebased 👍

[cincuranet]

Thanks. Table in #19635 updated.