SQLite

  if( sqlite3ExprCompare(pA->pRight, pB->pRight) ) return 2;
  if( sqlite3ExprListCompare(pA->x.pList, pB->x.pList) ) return 2;
  if( pA->iTable!=pB->iTable || pA->iColumn!=pB->iColumn ) return 2;
  if( ExprHasProperty(pA, EP_IntValue) ){
    if( !ExprHasProperty(pB, EP_IntValue) || pA->u.iValue!=pB->u.iValue ){
      return 2;
    }
  }else if( pA->op!=TK_COLUMN && pA->op!=TK_AGG_COLUMN && pA->u.zToken ){
    if( ExprHasProperty(pB, EP_IntValue) || NEVER(pB->u.zToken==0) ) return 2;
    if( strcmp(pA->u.zToken,pB->u.zToken)!=0 ){
      return 2;
    }
  }
  if( (pA->flags & EP_ExpCollate)!=(pB->flags & EP_ExpCollate) ) return 1;
  if( (pA->flags & EP_ExpCollate)!=0 && pA->pColl!=pB->pColl ) return 2;

    Expr *pExprA = pA->a[i].pExpr;
    Expr *pExprB = pB->a[i].pExpr;
    if( pA->a[i].sortOrder!=pB->a[i].sortOrder ) return 1;
    if( sqlite3ExprCompare(pExprA, pExprB) ) return 1;
  }
  return 0;
}

/*
** This is the expression callback for sqlite3FunctionUsesOtherSrc().
**
** Determine if an expression references any table other than one of the
** tables in pWalker->u.pSrcList and abort if it does.
*/
static int exprUsesOtherSrc(Walker *pWalker, Expr *pExpr){
  if( pExpr->op==TK_COLUMN || pExpr->op==TK_AGG_COLUMN ){
    int i;
    SrcList *pSrc = pWalker->u.pSrcList;
    for(i=0; i<pSrc->nSrc; i++){
      if( pExpr->iTable==pSrc->a[i].iCursor ) return WRC_Continue;
    }
    return WRC_Abort;
  }else{
    return WRC_Continue;
  }
}

/*
** Determine if any of the arguments to the pExpr Function references
** any SrcList other than pSrcList.  Return true if they do.  Return
** false if pExpr has no argument or has only constant arguments or
** only references tables named in pSrcList.
*/
static int sqlite3FunctionUsesOtherSrc(Expr *pExpr, SrcList *pSrcList){
  Walker w;
  assert( pExpr->op==TK_AGG_FUNCTION );
  memset(&w, 0, sizeof(w));
  w.xExprCallback = exprUsesOtherSrc;
  w.u.pSrcList = pSrcList;
  if( sqlite3WalkExprList(&w, pExpr->x.pList)!=WRC_Continue ) return 1;
  return 0;
}

/*
** Add a new element to the pAggInfo->aCol[] array.  Return the index of
** the new element.  Return a negative number if malloc fails.
*/
static int addAggInfoColumn(sqlite3 *db, AggInfo *pInfo){
  int i;

            break;
          } /* endif pExpr->iTable==pItem->iCursor */
        } /* end loop over pSrcList */
      }
      return WRC_Prune;
    }
    case TK_AGG_FUNCTION: {
      if( !sqlite3FunctionUsesOtherSrc(pExpr, pSrcList) ){


        /* Check to see if pExpr is a duplicate of another aggregate 
        ** function that is already in the pAggInfo structure
        */
        struct AggInfo_func *pItem = pAggInfo->aFunc;
        for(i=0; i<pAggInfo->nFunc; i++, pItem++){
          if( sqlite3ExprCompare(pItem->pExpr, pExpr)==0 ){
            break;

        return WRC_Prune;
      }
    }
  }
  return WRC_Continue;
}
static int analyzeAggregatesInSelect(Walker *pWalker, Select *pSelect){







  return WRC_Continue;

}

/*
** Analyze the given expression looking for aggregate functions and
** for variables that need to be added to the pParse->aAgg[] array.
** Make additional entries to the pParse->aAgg[] array 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.u.pNC = pNC;
  assert( pNC->pSrcList!=0 );
  sqlite3WalkExpr(&w, pExpr);
}

  SrcList *pSrcList;   /* One or more tables used to resolve names */
  ExprList *pEList;    /* Optional list of named expressions */
  int nRef;            /* Number of names resolved by this context */
  int nErr;            /* Number of errors encountered while resolving names */
  u8 allowAgg;         /* Aggregate functions allowed here */
  u8 hasAgg;           /* True if aggregates are seen */
  u8 isCheck;          /* True if resolving names in a CHECK constraint */

  AggInfo *pAggInfo;   /* Information about aggregates at this level */
  NameContext *pNext;  /* Next outer name context.  NULL for outermost */
};

/*
** An instance of the following structure contains all information
** needed to generate code for a single SELECT statement.

struct Walker {
  int (*xExprCallback)(Walker*, Expr*);     /* Callback for expressions */
  int (*xSelectCallback)(Walker*,Select*);  /* Callback for SELECTs */
  Parse *pParse;                            /* Parser context.  */
  union {                                   /* Extra data for callback */
    NameContext *pNC;                          /* Naming context */
    int i;                                     /* Integer value */
    SrcList *pSrcList;                         /* FROM clause */
  } u;
};

/* Forward declarations */
int sqlite3WalkExpr(Walker*, Expr*);
int sqlite3WalkExprList(Walker*, ExprList*);
int sqlite3WalkSelect(Walker*, Select*);

  }
} {1 one 2 two}
do_test subquery-3.3.5 {
  execsql {
    SELECT a, (SELECT count(*) FROM t2 WHERE a=c) FROM t1;
  }
} {1 1 2 1}

# The following tests check for aggregate subqueries in an aggregate
# query.
#
do_test subquery-3.4.1 {
  execsql {
    CREATE TABLE t34(x,y);
    INSERT INTO t34 VALUES(106,4), (107,3), (106,5), (107,5);
    SELECT a.x, avg(a.y)
      FROM t34 AS a
     GROUP BY a.x
     HAVING NOT EXISTS( SELECT b.x, avg(b.y)
                          FROM t34 AS b
                         GROUP BY b.x
                         HAVING avg(a.y) > avg(b.y));
  }
} {107 4.0}
do_test subquery-3.4.2 {
  execsql {
    SELECT a.x, avg(a.y) AS avg1
      FROM t34 AS a
     GROUP BY a.x
     HAVING NOT EXISTS( SELECT b.x, avg(b.y) AS avg2
                          FROM t34 AS b
                         GROUP BY b.x
                         HAVING avg1 > avg2);
  }
} {107 4.0}
do_test subquery-3.4.3 {
  execsql {
    SELECT
       a.x,
       avg(a.y),
       NOT EXISTS ( SELECT b.x, avg(b.y)
                      FROM t34 AS b
                      GROUP BY b.x
                     HAVING avg(a.y) > avg(b.y)),
       EXISTS ( SELECT c.x, avg(c.y)
                  FROM t34 AS c
                  GROUP BY c.x
                 HAVING avg(a.y) > avg(c.y))
      FROM t34 AS a
     GROUP BY a.x
     ORDER BY a.x;
  }
} {106 4.5 0 1 107 4.0 1 0}


#------------------------------------------------------------------
# These tests - subquery-4.* - use the TCL statement cache to try 
# and expose bugs to do with re-using statements that have been 
# passed to sqlite3_reset().
#
# One problem was that VDBE memory cells were not being initialised