private String findSubqAlias(Select sel) {
Select pSel = sel.getParent();
if (pSel == null)
return null;
Context pCtx = pSel.ctx();
if (pCtx.subquery == null)
return null;
if (pCtx.getSchema(_schemaAlias) != null)
return ((SubQ)pCtx.subquery).getCandidateAlias();
return findSubqAlias(pSel);