public JoinTable GetOrCreate(ReferenceInfo info,JoinTable prev=null) {
JoinTable result = null;
for (var i = 0; i < this._tables.Count; i++) {
result = this._tables[i];
if (result.ReferenceInfo == info && result.JoinFrom == prev) return result;
}
result = new JoinTable("_TMP_Tb" + (this._tables.Count + 1).ToString(),info,prev);
return result;
}
public JoinTable(string alias, ReferenceInfo info,JoinTable prev=null) {
this.Alias = alias;
this.ReferenceInfo = info;
this.JoinFrom = prev;
}