// INNER JOIN Products ON Categories.CategoryId > Products.CategoryId
// OR (Categories.CategoryId = Products.CategoryId
// AND Categories.CategoryId = Products.ProductId)

var join = new Join(api.PageInfo)
{
  EntityFromName = "Categories",
  EntityToName = "Products",
  Type = (int)JoinType.Inner,
};

join.JoinColumns.Add(new JoinColumn(
  new KeyColumn(join.EntityFromName, "CategoryId", JoinExpressionType.Column),
  JoinComparison.GT,
  new KeyColumn(join.EntityToName, "ProductId", JoinExpressionType.Column),
  JoinConjunction.OR,
  0 // nesting level - number of parens surrounding the expression
  ));

join.JoinColumns.Add(new JoinColumn(
  new KeyColumn(join.EntityFromName, "CategoryId", JoinExpressionType.Column),
  JoinComparison.EQ,
  new KeyColumn(join.EntityToName, "CategoryId", JoinExpressionType.Column),
  JoinConjunction.AND,
  1
  ));

join.JoinColumns.Add(new JoinColumn(
  new KeyColumn(join.EntityFromName, "CategoryId", JoinExpressionType.Column),
  JoinComparison.EQ,
  new KeyColumn(join.EntityToName, "ProductId", JoinExpressionType.Column),
  JoinConjunction.OR,
  1
  ));

// api.Joins.Add(join);    // add at the application level
// report.Joins.Add(join); // or at the report level

Entity fromEntity = api.Entities.GetEntity("fromName");
Entity toEntity = api.Entities.GetEntity("toName");

Join newJoin = new Join(api.PageInfo)
{
  EntityFromName = fromEntity.Name,
  EntityToName   = toEntity.Name,
  Type           = (int)JoinType.Inner,
  RelationType   = 0, // 0: One-to-One, 1: One-to-Many
  Weight         = 0
};

// Add key columns
newJoin.JoinColumns.Add(new JoinColumn(
 new KeyColumn(fromEntity.Name, fromEntity.GetColumn("fromColName").ActualFullName),
 new KeyColumn(toEntity.Name, toEntity.GetColumn("toColName").ActualFullName)
 ));

// Add to the config; for reports, use report.Joins.Add()
api.Joins.Add(newJoin);

// If there is an active report, recreate the joins
report.CreateJoins();