/// <summary>
/// 访问 <see cref="OuterJoinedExpression"/>。
/// </summary>
/// <param name="outer">要访问的表达式。</param>
/// <returns></returns>
protected virtual Expression VisitOuterJoined(OuterJoinedExpression outer)
{
var test = Visit(outer.Test);
var expression = Visit(outer.Expression);
return(outer.Update(test, expression));
}
internal static ProjectionExpression AddOuterJoinTest(this ProjectionExpression proj, string name = null)
{
var test = GetOuterJoinTest(proj.Select);
var select = proj.Select;
ColumnExpression testCol = null;
// look to see if test expression exists in columns already
foreach (var col in select.Columns)
{
if (test.Equals(col.Expression))
{
testCol = new ColumnExpression(test.Type, select.Alias, col.Name, null);
break;
}
}
if (testCol == null)
{
// add expression to projection
testCol = test as ColumnExpression;
string colName = (testCol != null) ? testCol.Name : "Test";
colName = proj.Select.Columns.GetAvailableColumnName(colName);
select = select.AddColumn(new ColumnDeclaration(colName, test));
testCol = new ColumnExpression(test.Type, select.Alias, colName, null);
}
var newProjector = new OuterJoinedExpression(testCol, proj.Projector);
return(new ProjectionExpression(select, newProjector, proj.Aggregator));
}