improve sql wall, support rand

src/main/java/com/alibaba/druid/sql/visitor/SQLEvalVisitor.java

@@ -30,6 +30,8 @@ public interface SQLEvalVisitor extends SQLASTVisitor {
     Function getFunction(String funcName);
 
     void registerFunction(String funcName, Function function);
+    
+    void unregisterFunction(String funcName);
 
     List<Object> getParameters();
 

src/main/java/com/alibaba/druid/sql/visitor/SQLEvalVisitorImpl.java

@@ -138,4 +138,9 @@ public class SQLEvalVisitorImpl extends SQLASTVisitorAdapter implements SQLEvalV
     public boolean visit(SQLIdentifierExpr x) {
         return SQLEvalVisitorUtils.visit(this, x);
     }
+
+    @Override
+    public void unregisterFunction(String funcName) {
+        functions.remove(funcName);
+    }
 }

src/main/java/com/alibaba/druid/sql/visitor/SQLEvalVisitorUtils.java

@@ -228,7 +228,11 @@ public class SQLEvalVisitorUtils {
     public static boolean visit(SQLEvalVisitor visitor, SQLMethodInvokeExpr x) {
         String methodName = x.getMethodName().toLowerCase();
 
-        Function function = functions.get(methodName);
+        Function function = visitor.getFunction(methodName);
+
+        if (function == null) {
+            function = functions.get(methodName);
+        }
 
         if (function != null) {
             Object result = function.eval(visitor, x);

src/main/java/com/alibaba/druid/sql/visitor/functions/Nil.java

@@ -0,0 +1,15 @@
+package com.alibaba.druid.sql.visitor.functions;
+
+import com.alibaba.druid.sql.ast.expr.SQLMethodInvokeExpr;
+import com.alibaba.druid.sql.visitor.SQLEvalVisitor;
+
+public class Nil implements Function {
+
+    public final static Nil instance = new Nil();
+
+    @Override
+    public Object eval(SQLEvalVisitor visitor, SQLMethodInvokeExpr x) {
+        return null;
+    }
+
+}