improved sql parser.

src/main/java/com/alibaba/druid/sql/dialect/mysql/ast/expr/MySqlExtractExpr.java

@@ -28,11 +28,23 @@ public class MySqlExtractExpr extends SQLExprImpl implements MySqlExpr {
     public MySqlExtractExpr(){
     }
 
+    public MySqlExtractExpr clone() {
+        MySqlExtractExpr x = new MySqlExtractExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        x.unit = unit;
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }
 
     public void setValue(SQLExpr value) {
+        if (value != null) {
+            value.setParent(this);
+        }
         this.value = value;
     }
 

src/main/java/com/alibaba/druid/sql/dialect/mysql/ast/expr/MySqlIntervalExpr.java

@@ -28,6 +28,15 @@ public class MySqlIntervalExpr extends SQLExprImpl implements MySqlExpr {
     public MySqlIntervalExpr(){
     }
 
+    public MySqlIntervalExpr clone() {
+        MySqlIntervalExpr x = new MySqlIntervalExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        x.unit = unit;
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/mysql/ast/expr/MySqlMatchAgainstExpr.java

@@ -31,6 +31,20 @@ public class MySqlMatchAgainstExpr extends SQLExprImpl implements MySqlExpr {
 
     private SearchModifier searchModifier;
 
+    public MySqlMatchAgainstExpr clone() {
+        MySqlMatchAgainstExpr x = new MySqlMatchAgainstExpr();
+        for (SQLExpr column : columns) {
+            SQLExpr column2 = column.clone();
+            column2.setParent(x);
+            x.columns.add(column2);
+        }
+        if (against != null) {
+            x.setAgainst(against.clone());
+        }
+        x.searchModifier = searchModifier;
+        return x;
+    }
+
     public List<SQLExpr> getColumns() {
         return columns;
     }
@@ -44,6 +58,9 @@ public class MySqlMatchAgainstExpr extends SQLExprImpl implements MySqlExpr {
     }
 
     public void setAgainst(SQLExpr against) {
+        if (against != null) {
+            against.setParent(this);
+        }
         this.against = against;
     }
 

src/main/java/com/alibaba/druid/sql/dialect/mysql/ast/expr/MySqlOrderingExpr.java

@@ -32,10 +32,19 @@ public class MySqlOrderingExpr extends SQLExprImpl implements MySqlExpr {
     
     public MySqlOrderingExpr(SQLExpr expr, SQLOrderingSpecification type){
         super();
-        this.expr = expr;
+        setExpr(expr);
         this.type = type;
     }
 
+    public MySqlOrderingExpr clone() {
+        MySqlOrderingExpr x = new MySqlOrderingExpr();
+        if (expr != null) {
+            x.setExpr(expr.clone());
+        }
+        x.type = type;
+        return x;
+    }
+
     @Override
     protected void accept0(SQLASTVisitor visitor) {
         MySqlASTVisitor mysqlVisitor = (MySqlASTVisitor) visitor;
@@ -51,7 +60,9 @@ public class MySqlOrderingExpr extends SQLExprImpl implements MySqlExpr {
     }
 
     public void setExpr(SQLExpr expr) {
-        expr.setParent(this);
+        if (expr != null) {
+            expr.setParent(this);
+        }
         this.expr = expr;
     }
 

src/main/java/com/alibaba/druid/sql/dialect/mysql/ast/statement/MySqlSelectQueryBlock.java

@@ -22,6 +22,7 @@ import com.alibaba.druid.sql.ast.*;
 import com.alibaba.druid.sql.ast.statement.SQLSelectQueryBlock;
 import com.alibaba.druid.sql.dialect.mysql.ast.MySqlObject;
 import com.alibaba.druid.sql.dialect.mysql.visitor.MySqlASTVisitor;
+import com.alibaba.druid.sql.dialect.oracle.ast.stmt.OracleSelectQueryBlock;
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;
 
 public class MySqlSelectQueryBlock extends SQLSelectQueryBlock implements MySqlObject {
@@ -46,6 +47,34 @@ public class MySqlSelectQueryBlock extends SQLSelectQueryBlock implements MySqlO
 
     }
 
+    public MySqlSelectQueryBlock clone() {
+        MySqlSelectQueryBlock x = new MySqlSelectQueryBlock();
+        cloneTo(x);
+
+        x.hignPriority = hignPriority;
+        x.straightJoin = straightJoin;
+
+        x.smallResult = smallResult;
+        x.bigResult = bigResult;
+        x.bufferResult = bufferResult;
+        x.cache = cache;
+        x.calcFoundRows = calcFoundRows;
+
+        if (procedureName != null) {
+            x.setProcedureName(procedureName.clone());
+        }
+        if (procedureArgumentList != null) {
+            for (SQLExpr arg : procedureArgumentList) {
+                SQLExpr arg_cloned = arg.clone();
+                arg_cloned.setParent(this);
+                x.procedureArgumentList.add(arg_cloned);
+            }
+        }
+        x.lockInShareMode = lockInShareMode;
+
+        return x;
+    }
+
     public int getHintsSize() {
         if (hints == null) {
             return 0;

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

@@ -16,10 +16,7 @@
 package com.alibaba.druid.sql.dialect.mysql.visitor;
 
 import com.alibaba.druid.sql.ast.*;
-import com.alibaba.druid.sql.ast.expr.SQLAggregateExpr;
-import com.alibaba.druid.sql.ast.expr.SQLCharExpr;
-import com.alibaba.druid.sql.ast.expr.SQLMethodInvokeExpr;
-import com.alibaba.druid.sql.ast.expr.SQLVariantRefExpr;
+import com.alibaba.druid.sql.ast.expr.*;
 import com.alibaba.druid.sql.ast.statement.*;
 import com.alibaba.druid.sql.dialect.mysql.ast.MySqlForceIndexHint;
 import com.alibaba.druid.sql.dialect.mysql.ast.MySqlIgnoreIndexHint;
@@ -3677,4 +3674,31 @@ public class MySqlOutputVisitor extends SQLASTOutputVisitor implements MySqlASTV
         block.accept(this);
         return false;
     }
+
+    @Override
+    public boolean visit(SQLCommentStatement x) {
+        SQLCommentStatement.Type type = x.getType();
+
+        SQLExpr on = x.getOn();
+        if (type == SQLCommentStatement.Type.TABLE) {
+            print0(ucase ? "ALTER TABLE " : "alter table ");
+            on.accept(this);
+            print0(ucase ? " COMMENT = " : " comment = ");
+            x.getComment().accept(this);
+        } else {
+            SQLPropertyExpr propertyExpr = (SQLPropertyExpr) on;
+
+            SQLExpr table = propertyExpr.getOwner();
+            String column = propertyExpr.getName();
+
+            print0(ucase ? "ALTER TABLE " : "alter table ");
+            table.accept(this);
+            print0(ucase ? " MODIFY COLUMN " : " modify column ");
+            print(column);
+            print0(ucase ? " COMMENT " : " comment ");
+            x.getComment().accept(this);
+        }
+
+        return false;
+    }
 } //

src/main/java/com/alibaba/druid/sql/dialect/oracle/ast/expr/OracleCursorExpr.java

@@ -28,9 +28,16 @@ public class OracleCursorExpr extends SQLExprImpl implements OracleExpr {
 
     }
 
+    public OracleCursorExpr clone() {
+        OracleCursorExpr x = new OracleCursorExpr();
+        if (query != null) {
+            x.setQuery(query.clone());
+        }
+        return x;
+    }
+
     public OracleCursorExpr(SQLSelect query){
-        super();
-        this.query = query;
+        setQuery(query);
     }
 
     public SQLSelect getQuery() {

src/main/java/com/alibaba/druid/sql/dialect/oracle/ast/expr/OracleIsSetExpr.java

@@ -31,11 +31,22 @@ public class OracleIsSetExpr extends SQLExprImpl implements OracleExpr {
         this.nestedTable = nestedTable;
     }
 
+    public OracleIsSetExpr clone() {
+        OracleIsSetExpr x = new OracleIsSetExpr();
+        if (nestedTable != null) {
+            x.setNestedTable(nestedTable.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getNestedTable() {
         return nestedTable;
     }
 
     public void setNestedTable(SQLExpr nestedTable) {
+        if (nestedTable != null) {
+            nestedTable.setParent(this);
+        }
         this.nestedTable = nestedTable;
     }
 

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGBoxExpr.java

@@ -22,6 +22,14 @@ public class PGBoxExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGBoxExpr clone() {
+        PGBoxExpr x = new PGBoxExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGCidrExpr.java

@@ -22,6 +22,14 @@ public class PGCidrExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGCidrExpr clone() {
+        PGCidrExpr x = new PGCidrExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGCircleExpr.java

@@ -22,6 +22,14 @@ public class PGCircleExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGCircleExpr clone() {
+        PGCircleExpr x = new PGCircleExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGExtractExpr.java

@@ -23,6 +23,15 @@ public class PGExtractExpr extends PGExprImpl {
     private PGDateField field;
     private SQLExpr     source;
 
+    public PGExtractExpr clone() {
+        PGExtractExpr x = new PGExtractExpr();
+        x.field = field;
+        if (source != null) {
+            x.setSource(source.clone());
+        }
+        return x;
+    }
+
     public PGDateField getField() {
         return field;
     }
@@ -36,6 +45,9 @@ public class PGExtractExpr extends PGExprImpl {
     }
 
     public void setSource(SQLExpr source) {
+        if (source != null) {
+            source.setParent(this);
+        }
         this.source = source;
     }
 

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGInetExpr.java

@@ -22,6 +22,14 @@ public class PGInetExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGInetExpr clone() {
+        PGInetExpr x = new PGInetExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGLineSegmentsExpr.java

@@ -22,6 +22,14 @@ public class PGLineSegmentsExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGLineSegmentsExpr clone() {
+        PGLineSegmentsExpr x = new PGLineSegmentsExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGMacAddrExpr.java

@@ -23,6 +23,14 @@ public class PGMacAddrExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGMacAddrExpr clone() {
+        PGMacAddrExpr x = new PGMacAddrExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGPointExpr.java

@@ -22,6 +22,14 @@ public class PGPointExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGPointExpr clone() {
+        PGPointExpr x = new PGPointExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }

src/main/java/com/alibaba/druid/sql/dialect/postgresql/ast/expr/PGPolygonExpr.java

@@ -23,6 +23,14 @@ public class PGPolygonExpr extends PGExprImpl {
 
     private SQLExpr value;
 
+    public PGPolygonExpr clone() {
+        PGPolygonExpr x = new PGPolygonExpr();
+        if (value != null) {
+            x.setValue(value.clone());
+        }
+        return x;
+    }
+
     public SQLExpr getValue() {
         return value;
     }