SQLite

static int selectNodeIsConstant(Walker *pWalker, Select *NotUsed){
  UNUSED_PARAMETER(NotUsed);
  pWalker->eCode = 0;
  return WRC_Abort;
}
static int exprIsConst(Expr *p, int initFlag, int iCur){
  Walker w;
  memset(&w, 0, sizeof(w));
  w.eCode = initFlag;
  w.xExprCallback = exprNodeIsConstant;
  w.xSelectCallback = selectNodeIsConstant;
#ifdef SQLITE_DEBUG
  w.xSelectCallback2 = sqlite3SelectWalkAssert2;
#endif
  w.u.iCur = iCur;
  sqlite3WalkExpr(&w, p);
  return w.eCode;
}

/*
** Walk an expression tree.  Return non-zero if the expression is constant

** sequence as the GROUP BY term, but that is much harder to check,
** alternative collating sequences are uncommon, and this is only an
** optimization, so we take the easy way out and simply require the
** GROUP BY to use the BINARY collating sequence.
*/
int sqlite3ExprIsConstantOrGroupBy(Parse *pParse, Expr *p, ExprList *pGroupBy){
  Walker w;
  memset(&w, 0, sizeof(w));
  w.eCode = 1;
  w.xExprCallback = exprNodeIsConstantOrGroupBy;
  w.xSelectCallback = 0;
  w.u.pGroupBy = pGroupBy;
  w.pParse = pParse;
  sqlite3WalkExpr(&w, p);
  return w.eCode;
}

/*

#ifdef SQLITE_ENABLE_CURSOR_HINTS
/*
** Walk an expression tree.  Return 1 if the expression contains a
** subquery of some kind.  Return 0 if there are no subqueries.
*/
int sqlite3ExprContainsSubquery(Expr *p){
  Walker w;
  memset(&w, 0, sizeof(w));
  w.eCode = 1;
  w.xExprCallback = sqlite3ExprWalkNoop;
  w.xSelectCallback = selectNodeIsConstant;
#ifdef SQLITE_DEBUG
  w.xSelectCallback2 = sqlite3SelectWalkAssert2;
#endif
  sqlite3WalkExpr(&w, p);
  return w.eCode==0;
}
#endif

/*
** If the expression p codes a constant integer that is small enough

** has no arguments or has only constant arguments.  Return false if pExpr
** references columns but not columns of tables found in pSrcList.
*/
int sqlite3FunctionUsesThisSrc(Expr *pExpr, SrcList *pSrcList){
  Walker w;
  struct SrcCount cnt;
  assert( pExpr->op==TK_AGG_FUNCTION );
  memset(&w, 0, sizeof(w));
  w.xExprCallback = exprSrcCount;
  w.xSelectCallback = 0;
  w.u.pSrcCount = &cnt;
  cnt.pSrc = pSrcList;
  cnt.nThis = 0;
  cnt.nOther = 0;
  sqlite3WalkExprList(&w, pExpr->x.pList);
  return cnt.nThis>0 || cnt.nOther==0;
}

        return WRC_Continue;
      }
    }
  }
  return WRC_Continue;
}
static int analyzeAggregatesInSelect(Walker *pWalker, Select *pSelect){
  UNUSED_PARAMETER(pWalker);
  UNUSED_PARAMETER(pSelect);
  pWalker->walkerDepth++;
  return WRC_Continue;
}
static void analyzeAggregatesInSelectEnd(Walker *pWalker, Select *pSelect){
  UNUSED_PARAMETER(pSelect);
  pWalker->walkerDepth--;
}

/*
** Analyze the pExpr expression looking for aggregate functions and
** for variables that need to be added to AggInfo object that pNC->pAggInfo
** points to.  Additional entries are made on the AggInfo object as
** necessary.
**
** This routine should only be called after the expression has been
** analyzed by sqlite3ResolveExprNames().
*/
void sqlite3ExprAnalyzeAggregates(NameContext *pNC, Expr *pExpr){
  Walker w;
  memset(&w, 0, sizeof(w));
  w.xExprCallback = analyzeAggregate;
  w.xSelectCallback = analyzeAggregatesInSelect;
  w.xSelectCallback2 = analyzeAggregatesInSelectEnd;
  w.walkerDepth = 0;
  w.u.pNC = pNC;
  assert( pNC->pSrcList!=0 );
  sqlite3WalkExpr(&w, pExpr);
}

/*
** Call sqlite3ExprAnalyzeAggregates() for every expression in an

#endif
  savedHasAgg = pNC->ncFlags & (NC_HasAgg|NC_MinMaxAgg);
  pNC->ncFlags &= ~(NC_HasAgg|NC_MinMaxAgg);
  w.pParse = pNC->pParse;
  w.xExprCallback = resolveExprStep;
  w.xSelectCallback = resolveSelectStep;
  w.xSelectCallback2 = 0;
  w.walkerDepth = 0;
  w.eCode = 0;
  w.u.pNC = pNC;
  sqlite3WalkExpr(&w, pExpr);
#if SQLITE_MAX_EXPR_DEPTH>0
  pNC->pParse->nHeight -= pExpr->nHeight;
#endif
  if( pNC->nErr>0 || w.pParse->nErr>0 ){
    ExprSetProperty(pExpr, EP_Error);

  Parse *pParse,         /* The parser context */
  Select *p,             /* The SELECT statement being coded. */
  NameContext *pOuterNC  /* Name context for parent SELECT statement */
){
  Walker w;

  assert( p!=0 );
  memset(&w, 0, sizeof(w));
  w.xExprCallback = resolveExprStep;
  w.xSelectCallback = resolveSelectStep;
  w.xSelectCallback2 = 0;
  w.pParse = pParse;
  w.u.pNC = pOuterNC;
  sqlite3WalkSelect(&w, p);
}

/*
** Resolve names in expressions that can only reference a single table:

** subquery in the parser tree.
*/
int sqlite3ExprWalkNoop(Walker *NotUsed, Expr *NotUsed2){
  UNUSED_PARAMETER2(NotUsed, NotUsed2);
  return WRC_Continue;
}

/*
** No-op routine for the parse-tree walker for SELECT statements.
** subquery in the parser tree.
*/
int sqlite3SelectWalkNoop(Walker *NotUsed, Select *NotUsed2){
  UNUSED_PARAMETER2(NotUsed, NotUsed2);
  return WRC_Continue;
}

#if SQLITE_DEBUG
/*
** Always assert.  This xSelectCallback2 implementation proves that the
** xSelectCallback2 is never invoked.
*/
void sqlite3SelectWalkAssert2(Walker *NotUsed, Select *NotUsed2){
  UNUSED_PARAMETER2(NotUsed, NotUsed2);
  assert( 0 );
}
#endif
/*
** This routine "expands" a SELECT statement and all of its subqueries.
** For additional information on what it means to "expand" a SELECT
** statement, see the comment on the selectExpand worker callback above.
**
** Expanding a SELECT statement is the first step in processing a
** SELECT statement.  The SELECT statement must be expanded before
** name resolution is performed.
**
** If anything goes wrong, an error message is written into pParse.
** The calling function can detect the problem by looking at pParse->nErr
** and/or pParse->db->mallocFailed.
*/
static void sqlite3SelectExpand(Parse *pParse, Select *pSelect){
  Walker w;
  memset(&w, 0, sizeof(w));
  w.xExprCallback = sqlite3ExprWalkNoop;
  w.pParse = pParse;
  if( pParse->hasCompound ){
    w.xSelectCallback = convertCompoundSelectToSubquery;
    w.xSelectCallback2 = 0;
    sqlite3WalkSelect(&w, pSelect);
  }
  w.xSelectCallback = selectExpander;
  w.xSelectCallback2 = selectPopWith;
  sqlite3WalkSelect(&w, pSelect);
}

** SELECT statement.
**
** Use this routine after name resolution.
*/
static void sqlite3SelectAddTypeInfo(Parse *pParse, Select *pSelect){
#ifndef SQLITE_OMIT_SUBQUERY
  Walker w;
  memset(&w, 0, sizeof(w));
  w.xSelectCallback = sqlite3SelectWalkNoop;
  w.xSelectCallback2 = selectAddSubqueryTypeInfo;
  w.xExprCallback = sqlite3ExprWalkNoop;
  w.pParse = pParse;
  sqlite3WalkSelect(&w, pSelect);
#endif
}

/* Forward declarations */
int sqlite3WalkExpr(Walker*, Expr*);
int sqlite3WalkExprList(Walker*, ExprList*);
int sqlite3WalkSelect(Walker*, Select*);
int sqlite3WalkSelectExpr(Walker*, Select*);
int sqlite3WalkSelectFrom(Walker*, Select*);
int sqlite3ExprWalkNoop(Walker*, Expr*);
int sqlite3SelectWalkNoop(Walker*, Select*);
#ifdef SQLITE_DEBUG
void sqlite3SelectWalkAssert2(Walker*, Select*);
#endif

/*
** Return code from the parse-tree walking primitives and their
** callbacks.
*/
#define WRC_Continue    0   /* Continue down into children */
#define WRC_Prune       1   /* Omit children but continue walking siblings */

/*
** Call sqlite3WalkExpr() for every expression in Select statement p.
** Invoke sqlite3WalkSelect() for subqueries in the FROM clause and
** on the compound select chain, p->pPrior. 
**
** If it is not NULL, the xSelectCallback() callback is invoked before
** the walk of the expressions and FROM clause. The xSelectCallback2()
** method, if it is not NULL, is invoked following the walk of the 
** expressions and FROM clause.
** method is invoked following the walk of the expressions and FROM clause,
** but only if both xSelectCallback and xSelectCallback2 are both non-NULL
** and if the expressions and FROM clause both return WRC_Continue;
**
** Return WRC_Continue under normal conditions.  Return WRC_Abort if
** there is an abort request.
**
** If the Walker does not have an xSelectCallback() then this routine
** is a no-op returning WRC_Continue.
*/
int sqlite3WalkSelect(Walker *pWalker, Select *p){
  int rc;
  if( p==0 || (pWalker->xSelectCallback==0 && pWalker->xSelectCallback2==0) ){
    return WRC_Continue;
  if( p==0 || pWalker->xSelectCallback==0 ) return WRC_Continue;
  }
  rc = WRC_Continue;
  do{
  pWalker->walkerDepth++;
  while( p ){
    if( pWalker->xSelectCallback ){
       rc = pWalker->xSelectCallback(pWalker, p);
       if( rc ) break;
    rc = pWalker->xSelectCallback(pWalker, p);
    if( rc ) return rc & WRC_Abort;
    }
    if( sqlite3WalkSelectExpr(pWalker, p)
     || sqlite3WalkSelectFrom(pWalker, p)
    ){
      pWalker->walkerDepth--;
      return WRC_Abort;
    }
    if( pWalker->xSelectCallback2 ){
      pWalker->xSelectCallback2(pWalker, p);
    }
    p = p->pPrior;
  }
  }while( p!=0 );
  pWalker->walkerDepth--;
  return rc & WRC_Abort;
  return WRC_Continue;
}