mysql parser refactor

src/main/java/com/alibaba/druid/sql/parser/SQLExprParser.java

@@ -1163,6 +1163,12 @@ public class SQLExprParser extends SQLParser {
             column.getConstaints().add(new NotNullConstraint());
             return parseColumnRest(column);
         }
+        
+        if (lexer.token() == Token.NULL) {
+            lexer.nextToken();
+            column.setDefaultExpr(new SQLNullExpr());
+            return parseColumnRest(column);
+        }
 
         return column;
     }

src/main/java/com/alibaba/druid/sql/parser/SQLStatementParser.java

@@ -164,7 +164,7 @@ public class SQLStatementParser extends SQLParser {
         return stmt;
     }
     
-    public SQLSetStatement parseAlter() {
+    public SQLStatement parseAlter() {
         accept(Token.ALTER);
         throw new ParserException("TODO " + lexer.token() + " " + lexer.stringVal());
     }