improve sql wall, support rand

src/main/java/com/alibaba/druid/sql/dialect/db2/visitor/DB2EvalVisitor.java

@@ -135,6 +135,11 @@ public class DB2EvalVisitor extends DB2ASTVisitorAdapter implements SQLEvalVisit
     public void registerFunction(String funcName, Function function) {
         functions.put(funcName, function);
     }
+    
+    @Override
+    public void unregisterFunction(String funcName) {
+        functions.remove(funcName);
+    }
 
     public boolean visit(SQLIdentifierExpr x) {
         return SQLEvalVisitorUtils.visit(this, x);

src/main/java/com/alibaba/druid/sql/dialect/mysql/visitor/MySqlEvalVisitorImpl.java

@@ -162,4 +162,9 @@ public class MySqlEvalVisitorImpl extends MySqlASTVisitorAdapter implements SQLE
     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/dialect/oracle/visitor/OracleEvalVisitor.java

@@ -135,6 +135,11 @@ public class OracleEvalVisitor extends OracleASTVisitorAdapter implements SQLEva
     public void registerFunction(String funcName, Function function) {
         functions.put(funcName, function);
     }
+    
+    @Override
+    public void unregisterFunction(String funcName) {
+        functions.remove(funcName);
+    }
 
     public boolean visit(SQLIdentifierExpr x) {
         return SQLEvalVisitorUtils.visit(this, x);

src/main/java/com/alibaba/druid/sql/dialect/postgresql/visitor/PGEvalVisitor.java

@@ -135,6 +135,11 @@ public class PGEvalVisitor extends PGASTVisitorAdapter implements SQLEvalVisitor
     public void registerFunction(String funcName, Function function) {
         functions.put(funcName, function);
     }
+    
+    @Override
+    public void unregisterFunction(String funcName) {
+        functions.remove(funcName);
+    }
 
     public boolean visit(SQLIdentifierExpr x) {
         return SQLEvalVisitorUtils.visit(this, x);

src/main/java/com/alibaba/druid/sql/dialect/sqlserver/visitor/SQLServerEvalVisitor.java

@@ -136,6 +136,11 @@ public class SQLServerEvalVisitor extends SQLServerASTVisitorAdapter implements
         functions.put(funcName, function);
     }
     
+    @Override
+    public void unregisterFunction(String funcName) {
+        functions.remove(funcName);
+    }
+    
     public boolean visit(SQLIdentifierExpr x) {
         return SQLEvalVisitorUtils.visit(this, x);
     }