Index: src/window.c
==================================================================
--- src/window.c
+++ src/window.c
@@ -409,18 +409,23 @@
     pWin->eStart = p->eStart;
     pWin->eEnd = p->eEnd;
   }
   if( pFunc->funcFlags & SQLITE_FUNC_WINDOW ){
     sqlite3 *db = pParse->db;
+    if( pWin->pFilter ){
+      sqlite3ErrorMsg(pParse, 
+          "FILTER clause may only be used with aggregate window functions"
+      );
+    }else
     if( pFunc->xSFunc==row_numberStepFunc || pFunc->xSFunc==ntileStepFunc ){
       sqlite3ExprDelete(db, pWin->pStart);
       sqlite3ExprDelete(db, pWin->pEnd);
       pWin->pStart = pWin->pEnd = 0;
       pWin->eType = TK_ROWS;
       pWin->eStart = TK_UNBOUNDED;
       pWin->eEnd = TK_CURRENT;
-    }
+    }else
 
     if( pFunc->xSFunc==dense_rankStepFunc || pFunc->xSFunc==rankStepFunc
      || pFunc->xSFunc==percent_rankStepFunc || pFunc->xSFunc==cume_distStepFunc
     ){
       sqlite3ExprDelete(db, pWin->pStart);
@@ -600,10 +605,14 @@
     ** window function - one for the accumulator, another for interim
     ** results.  */
     for(pWin=pMWin; pWin; pWin=pWin->pNextWin){
       pWin->iArgCol = (pSublist ? pSublist->nExpr : 0);
       pSublist = exprListAppendList(pParse, pSublist, pWin->pOwner->x.pList);
+      if( pWin->pFilter ){
+        Expr *pFilter = sqlite3ExprDup(db, pWin->pFilter, 0);
+        pSublist = sqlite3ExprListAppend(pParse, pSublist, pFilter);
+      }
       pWin->regAccum = ++pParse->nMem;
       pWin->regResult = ++pParse->nMem;
       sqlite3VdbeAddOp2(v, OP_Null, 0, pWin->regAccum);
     }
 
@@ -817,18 +826,33 @@
     }else if( pWin->pFunc->xSFunc==leadStepFunc 
            || pWin->pFunc->xSFunc==lagStepFunc 
     ){
       /* no-op */
     }else{
+      int addrIf = 0;
+      if( pWin->pFilter ){
+        int regTmp;
+        if( csr>0 ){
+          regTmp = sqlite3GetTempReg(pParse);
+          sqlite3VdbeAddOp3(v, OP_Column, csr, pWin->iArgCol+pWin->nArg,regTmp);
+        }else{
+          regTmp = regArg + pWin->nArg;
+        }
+        addrIf = sqlite3VdbeAddOp3(v, OP_IfNot, regTmp, 0, 1);
+        if( csr>0 ){
+          sqlite3ReleaseTempReg(pParse, regTmp);
+        }
+      }
       if( pWin->pFunc->funcFlags & SQLITE_FUNC_NEEDCOLL ){
         CollSeq *pColl;
-        pColl = sqlite3ExprCollSeq(pParse, pWin->pOwner->x.pList->a[0].pExpr);
+        pColl = sqlite3ExprNNCollSeq(pParse, pWin->pOwner->x.pList->a[0].pExpr);
         sqlite3VdbeAddOp4(v, OP_CollSeq, 0,0,0, (const char*)pColl, P4_COLLSEQ);
       }
       sqlite3VdbeAddOp3(v, OP_AggStep0, bInverse, regArg, pWin->regAccum);
       sqlite3VdbeAppendP4(v, pWin->pFunc, P4_FUNCDEF);
       sqlite3VdbeChangeP5(v, (u8)nArg);
+      if( addrIf ) sqlite3VdbeJumpHere(v, addrIf);
     }
   }
 }
 
 static void windowAggFinal(Parse *pParse, Window *pMWin, int bFinal){
@@ -853,10 +877,11 @@
       }
       sqlite3VdbeAddOp2(v, OP_AggFinal, pWin->regAccum, pWin->nArg);
       sqlite3VdbeAppendP4(v, pWin->pFunc, P4_FUNCDEF);
       if( bFinal ){
         sqlite3VdbeAddOp2(v, OP_Copy, pWin->regAccum, pWin->regResult);
+        sqlite3VdbeAddOp2(v, OP_Null, 0, pWin->regAccum);
       }else{
         sqlite3VdbeChangeP3(v, -1, pWin->regResult);
       }
     }
   }

Index: test/window1.test
==================================================================
--- test/window1.test
+++ test/window1.test
@@ -212,7 +212,11 @@
 } {
   b 1 1 b 2 2 b 3 3 b 4 4 b 5 5 b 6 6 b 7 7
   a 1 1 a 2 2 a 3 3 a 4 4 a 5 5 a 6 6 a 7 7
 }
 
+do_catchsql_test 3.5 {
+  SELECT x, lag(x) FILTER (WHERE (x%2)=0) OVER w FROM t1 
+  WINDOW w AS (ORDER BY x)
+} {1 {FILTER clause may only be used with aggregate window functions}}
 
 finish_test

Index: test/window3.tcl
==================================================================
--- test/window3.tcl
+++ test/window3.tcl
@@ -282,11 +282,10 @@
   "
 
   execsql_test 1.$tn.14.1 "
     SELECT string_agg(CAST(b AS TEXT), '.') OVER (ORDER BY a $window) FROM t2
   "
-
   execsql_test 1.$tn.14.2 "
     SELECT string_agg(CAST(b AS TEXT), '.') OVER (PARTITION BY b%10 ORDER BY a $window) FROM t2
   "
   execsql_test 1.$tn.14.3 "
     SELECT string_agg(CAST(b AS TEXT), '.') OVER ( ORDER BY b,a $window ) FROM t2
@@ -298,9 +297,29 @@
     SELECT string_agg(CAST(b AS TEXT), '.') OVER ( ORDER BY b%10,a $window ) FROM t2
   "
   execsql_test 1.$tn.14.6 "
     SELECT string_agg(CAST(b AS TEXT), '.') OVER (PARTITION BY b%2,a ORDER BY b%10 $window) FROM t2
   "
+
+  execsql_test 1.$tn.15.1 "
+    SELECT string_agg(CAST(b AS TEXT), '.') 
+    FILTER (WHERE a%2=0) OVER (ORDER BY a $window) FROM t2
+  "
+
+  execsql_test 1.$tn.15.2 "
+    SELECT string_agg(CAST(b AS TEXT), '.') 
+    FILTER (WHERE 0=1) OVER (ORDER BY a $window) FROM t2
+  "
+
+  execsql_test 1.$tn.15.3 "
+    SELECT string_agg(CAST(b AS TEXT), '.') 
+    FILTER (WHERE 1=0) OVER (PARTITION BY (a%10) ORDER BY a $window) FROM t2
+  "
+
+  execsql_test 1.$tn.15.4 "
+    SELECT string_agg(CAST(b AS TEXT), '.') 
+    FILTER (WHERE a%2=0) OVER (PARTITION BY (a%10) ORDER BY a $window) FROM t2
+  "
 }
 
 finish_test
 

Index: test/window3.test
==================================================================
--- test/window3.test
+++ test/window3.test
cannot compute difference between binary files

Index: test/window4.tcl
==================================================================
--- test/window4.tcl
+++ test/window4.tcl
@@ -94,8 +94,14 @@
   SELECT a, count(*) OVER abc, count(*) OVER def FROM t5
   WINDOW abc AS (ORDER BY a), 
          def AS (ORDER BY a DESC)
   ORDER BY a;
 }
+
+execsql_test 3.4 {
+  SELECT a, max(a) FILTER (WHERE (a%2)=0) OVER w FROM t5 
+  WINDOW w AS (ORDER BY a)
+}
+
 
 finish_test
 

Index: test/window4.test
==================================================================
--- test/window4.test
+++ test/window4.test
@@ -174,6 +174,16 @@
   WINDOW abc AS (ORDER BY a), 
          def AS (ORDER BY a DESC)
   ORDER BY a;
 } {1 1 5   2 2 4   3 3 3   4 4 2   5 5 1}
 
+explain_i {
+  SELECT a, max(a) FILTER (WHERE (a%2)=0) OVER w FROM t5 
+  WINDOW w AS (ORDER BY a)
+  }
+  breakpoint
+do_execsql_test 3.4 {
+  SELECT a, max(a) FILTER (WHERE (a%2)=0) OVER w FROM t5 
+  WINDOW w AS (ORDER BY a)
+} {1 {}   2 2   3 2   4 4   5 4}
+
 finish_test