improve mysql sql parser ddl support

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

@@ -0,0 +1,27 @@
+package com.alibaba.druid.sql.ast.statement;
+
+import com.alibaba.druid.sql.ast.SQLObjectImpl;
+import com.alibaba.druid.sql.visitor.SQLASTVisitor;
+
+public class SQLAlterTableAddPrimaryKey extends SQLObjectImpl implements SQLAlterTableItem {
+
+    private static final long serialVersionUID = 1L;
+    private SQLPrimaryKey     primaryKey;
+
+    public SQLPrimaryKey getPrimaryKey() {
+        return primaryKey;
+    }
+
+    public void setPrimaryKey(SQLPrimaryKey primaryKey) {
+        this.primaryKey = primaryKey;
+    }
+
+    @Override
+    protected void accept0(SQLASTVisitor visitor) {
+        if (visitor.visit(this)) {
+            acceptChild(visitor, primaryKey);
+        }
+        visitor.endVisit(this);
+    }
+
+}

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

@@ -0,0 +1,29 @@
+package com.alibaba.druid.sql.ast.statement;
+
+import com.alibaba.druid.sql.ast.SQLName;
+import com.alibaba.druid.sql.ast.SQLObjectImpl;
+import com.alibaba.druid.sql.visitor.SQLASTVisitor;
+
+public class SQLAlterTableDropIndex extends SQLObjectImpl implements SQLAlterTableItem {
+
+    private static final long serialVersionUID = 1L;
+
+    private SQLName           indexName;
+
+    public SQLName getIndexName() {
+        return indexName;
+    }
+
+    public void setIndexName(SQLName indexName) {
+        this.indexName = indexName;
+    }
+
+    @Override
+    protected void accept0(SQLASTVisitor visitor) {
+        if (visitor.visit(this)) {
+            acceptChild(visitor, indexName);
+        }
+        visitor.endVisit(this);
+    }
+
+}