improved sql parser.

src/main/java/com/alibaba/druid/sql/ast/SQLExprImpl.java

@@ -25,9 +25,7 @@ public abstract class SQLExprImpl extends SQLObjectImpl implements SQLExpr {
 
     public abstract int hashCode();
 
-    public SQLExpr clone() {
-        throw new UnsupportedOperationException(this.getClass().getName());
-    }
+    public abstract SQLExpr clone();
 
     public SQLDataType computeDataType() {
         return null;

src/main/java/com/alibaba/druid/sql/ast/expr/SQLAllExpr.java

@@ -28,8 +28,15 @@ public class SQLAllExpr extends SQLExprImpl {
     }
 
     public SQLAllExpr(SQLSelect select){
+        setSubQuery(select);
+    }
 
-        this.subQuery = select;
+    public SQLAllExpr clone() {
+        SQLAllExpr x = new SQLAllExpr();
+        if (subQuery != null) {
+            x.setSubQuery(subQuery.clone());
+        }
+        return x;
     }
 
     public SQLSelect getSubQuery() {
@@ -37,6 +44,9 @@ public class SQLAllExpr extends SQLExprImpl {
     }
 
     public void setSubQuery(SQLSelect subQuery) {
+        if (subQuery != null) {
+            subQuery.setParent(this);
+        }
         this.subQuery = subQuery;
     }
 

src/main/java/com/alibaba/druid/sql/ast/expr/SQLAnyExpr.java

@@ -33,8 +33,15 @@ public class SQLAnyExpr extends SQLExprImpl {
     }
 
     public SQLAnyExpr(SQLSelect select){
+        setSubQuery(select);
+    }
 
-        this.subQuery = select;
+    public SQLAnyExpr clone() {
+        SQLAnyExpr x = new SQLAnyExpr();
+        if (subQuery != null) {
+            x.setSubQuery(subQuery.clone());
+        }
+        return x;
     }
 
     public SQLSelect getSubQuery() {
@@ -42,6 +49,9 @@ public class SQLAnyExpr extends SQLExprImpl {
     }
 
     public void setSubQuery(SQLSelect subQuery) {
+        if (subQuery != null) {
+            subQuery.setParent(this);
+        }
         this.subQuery = subQuery;
     }
 

src/main/java/com/alibaba/druid/sql/ast/expr/SQLArrayExpr.java

@@ -28,6 +28,19 @@ public class SQLArrayExpr extends SQLExprImpl {
 
     private List<SQLExpr> values = new ArrayList<SQLExpr>();
 
+    public SQLArrayExpr clone() {
+        SQLArrayExpr x = new SQLArrayExpr();
+        if (expr != null) {
+            x.setExpr(expr.clone());
+        }
+        for (SQLExpr value : values) {
+            SQLExpr value2 = value.clone();
+            value2.setParent(x);
+            x.values.add(value2);
+        }
+        return x;
+    }
+
     public SQLExpr getExpr() {
         return expr;
     }

src/main/java/com/alibaba/druid/sql/ast/expr/SQLBetweenExpr.java

@@ -34,6 +34,21 @@ public class SQLBetweenExpr extends SQLExprImpl implements Serializable {
 
     }
 
+    public SQLBetweenExpr clone() {
+        SQLBetweenExpr x = new SQLBetweenExpr();
+        if (testExpr != null) {
+            x.setTestExpr(testExpr.clone());
+        }
+        x.not = not;
+        if (beginExpr != null) {
+            x.setBeginExpr(beginExpr.clone());
+        }
+        if (endExpr != null) {
+            x.setEndExpr(endExpr.clone());
+        }
+        return x;
+    }
+
     public SQLBetweenExpr(SQLExpr testExpr, SQLExpr beginExpr, SQLExpr endExpr){
         setTestExpr(testExpr);
         setBeginExpr(beginExpr);

src/main/java/com/alibaba/druid/sql/ast/expr/SQLCastExpr.java

@@ -97,4 +97,15 @@ public class SQLCastExpr extends SQLExprImpl {
     public SQLDataType computeDataType() {
         return dataType;
     }
+
+    public SQLCastExpr clone() {
+        SQLCastExpr x = new SQLCastExpr();
+        if (expr != null) {
+            x.setExpr(expr.clone());
+        }
+        if (dataType != null) {
+            x.setDataType(dataType.clone());
+        }
+        return x;
+    }
 }

src/main/java/com/alibaba/druid/sql/ast/expr/SQLCurrentOfCursorExpr.java

@@ -31,11 +31,22 @@ public class SQLCurrentOfCursorExpr extends SQLExprImpl {
         this.cursorName = cursorName;
     }
 
+    public SQLCurrentOfCursorExpr clone() {
+        SQLCurrentOfCursorExpr x = new SQLCurrentOfCursorExpr();
+        if (cursorName != null) {
+            x.setCursorName(cursorName.clone());
+        }
+        return x;
+    }
+
     public SQLName getCursorName() {
         return cursorName;
     }
 
     public void setCursorName(SQLName cursorName) {
+        if (cursorName != null) {
+            cursorName.setParent(this);
+        }
         this.cursorName = cursorName;
     }
 

src/main/java/com/alibaba/druid/sql/ast/expr/SQLGroupingSetExpr.java

@@ -20,12 +20,23 @@ import java.util.List;
 
 import com.alibaba.druid.sql.ast.SQLExpr;
 import com.alibaba.druid.sql.ast.SQLExprImpl;
+import com.alibaba.druid.sql.ast.statement.SQLExplainStatement;
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;
 
 public class SQLGroupingSetExpr extends SQLExprImpl {
 
     private final List<SQLExpr> parameters = new ArrayList<SQLExpr>();
 
+    public SQLGroupingSetExpr clone() {
+        SQLGroupingSetExpr x = new SQLGroupingSetExpr();
+        for (SQLExpr p : parameters) {
+            SQLExpr p2 = p.clone();
+            p2.setParent(x);
+            x.parameters.add(p2);
+        }
+        return x;
+    }
+
     public List<SQLExpr> getParameters() {
         return parameters;
     }

src/main/java/com/alibaba/druid/sql/ast/expr/SQLSomeExpr.java

@@ -33,8 +33,15 @@ public class SQLSomeExpr extends SQLExprImpl {
     }
 
     public SQLSomeExpr(SQLSelect select){
+        this.setSubQuery(select);
+    }
 
-        this.subQuery = select;
+    public SQLSomeExpr clone() {
+        SQLSomeExpr x = new SQLSomeExpr();
+        if (subQuery != null) {
+            x.setSubQuery(subQuery.clone());
+        }
+        return x;
     }
 
     public SQLSelect getSubQuery() {
@@ -42,6 +49,9 @@ public class SQLSomeExpr extends SQLExprImpl {
     }
 
     public void setSubQuery(SQLSelect subQuery) {
+        if (subQuery != null) {
+            subQuery.setParent(this);
+        }
         this.subQuery = subQuery;
     }
 

src/main/java/com/alibaba/druid/sql/ast/expr/SQLTimestampExpr.java

@@ -32,6 +32,14 @@ public class SQLTimestampExpr extends SQLExprImpl {
         
     }
 
+    public SQLTimestampExpr clone() {
+        SQLTimestampExpr x = new SQLTimestampExpr();
+        x.literal = literal;
+        x.timeZone = timeZone;
+        x.withTimeZone = withTimeZone;
+        return x;
+    }
+
     public String getLiteral() {
         return literal;
     }

src/main/java/com/alibaba/druid/sql/ast/expr/SQLUnaryExpr.java

@@ -36,6 +36,15 @@ public class SQLUnaryExpr extends SQLExprImpl implements Serializable {
         this.setExpr(expr);
     }
 
+    public SQLUnaryExpr clone() {
+        SQLUnaryExpr x = new SQLUnaryExpr();
+        if (expr != null) {
+            x.setExpr(expr.clone());
+        }
+        x.operator = operator;
+        return x;
+    }
+
     public SQLUnaryOperator getOperator() {
         return operator;
     }

src/main/java/com/alibaba/druid/sql/ast/statement/SQLCreateTableStatement.java

@@ -139,6 +139,9 @@ public class SQLCreateTableStatement extends SQLStatementImpl implements SQLDDLS
     }
 
     public void setSelect(SQLSelect select) {
+        if (select != null) {
+            select.setParent(this);
+        }
         this.select = select;
     }
 

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;
     }

src/main/java/com/alibaba/druid/sql/repository/SchemaRepository.java

@@ -30,6 +30,8 @@ import com.alibaba.druid.sql.dialect.oracle.ast.stmt.OracleSelectTableReference;
 import com.alibaba.druid.sql.dialect.oracle.visitor.OracleASTVisitorAdapter;
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;
 import com.alibaba.druid.sql.visitor.SQLASTVisitorAdapter;
+import com.alibaba.druid.support.logging.Log;
+import com.alibaba.druid.support.logging.LogFactory;
 import com.alibaba.druid.util.JdbcConstants;
 
 import java.io.IOException;
@@ -39,6 +41,7 @@ import java.util.*;
  * Created by wenshao on 03/06/2017.
  */
 public class SchemaRepository {
+    private static Log LOG = LogFactory.getLog(SchemaRepository.class);
     private Schema defaultSchema;
     protected String dbType;
     protected SQLASTVisitor consoleVisitor;
@@ -804,11 +807,12 @@ public class SchemaRepository {
     }
 
     boolean acceptCreateTable(SQLCreateTableStatement x) {
-        String schemaName = x.getSchema();
+        SQLCreateTableStatement x1 = x.clone();
+        String schemaName = x1.getSchema();
 
         Schema schema = findSchema(schemaName, true);
 
-        SQLSelect select = x.getSelect();
+        SQLSelect select = x1.getSelect();
         if (select != null) {
             select.accept(createResolveVisitor(SchemaResolveVisitor.Option.ResolveAllColumn));
 
@@ -822,21 +826,21 @@ public class SchemaRepository {
                     column.setName(name);
                     column.setDataType(dataType);
                     column.setDbType(dbType);
-                    x.getTableElementList().add(column);
+                    x1.getTableElementList().add(column);
                 }
-                x.setSelect(null);
+                x1.setSelect(null);
             }
         }
 
         x.setSchema(null);
 
-        String name = x.computeName();
+        String name = x1.computeName();
         SchemaObject table = schema.findTableOrView(name);
         if (table != null) {
-            return false;
+            LOG.info("replaced table '" + name + "'");
         }
 
-        table = new SchemaObjectImpl(name, SchemaObjectType.Table, x.clone());
+        table = new SchemaObjectImpl(name, SchemaObjectType.Table, x1);
         String name_lower = SQLUtils.normalize(name).toLowerCase();
         schema.objects.put(name_lower, table);
         return true;

src/main/java/com/alibaba/druid/util/StringUtils.java

@@ -122,6 +122,10 @@ public class StringUtils {
         return a.equalsIgnoreCase(b);
     }
 
+    public static boolean isEmpty(String value) {
+        return isEmpty((CharSequence) value);
+    }
+
     public static boolean isEmpty(CharSequence value) {
         if (value == null || value.length() == 0) {
             return true;