improve sqlwall

src/main/java/com/alibaba/druid/wall/spi/WallVisitorUtils.java

@@ -2153,7 +2153,18 @@ public class WallVisitorUtils {
             SQLTableSource from = queryBlock.getFrom();
 
             if (from == null) {
-                return true;
+                boolean itemIsConst = true;
+                for (SQLSelectItem item : queryBlock.getSelectList()) {
+                    if (item.getExpr() instanceof SQLIdentifierExpr || item.getExpr() instanceof SQLPropertyExpr) {
+                        itemIsConst = false;
+                        break;
+                    }
+                }
+                if (itemIsConst) {
+                    return true;
+                } else {
+                    return false;
+                }
             }
 
             if (from instanceof SQLExprTableSource) {

src/test/java/com/alibaba/druid/bvt/filter/wall/MySqlWallTest_union.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 1999-2011 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.druid.bvt.filter.wall;
+
+import junit.framework.TestCase;
+
+import org.junit.Assert;
+
+import com.alibaba.druid.wall.WallConfig;
+import com.alibaba.druid.wall.WallUtils;
+
+public class MySqlWallTest_union extends TestCase {
+
+    public void testUnion() throws Exception {
+        WallConfig config = new WallConfig();
+        config.setSelectUnionCheck(true);
+
+        Assert.assertFalse(WallUtils.isValidateMySql("select f1, f2 from t union select 1, 2", config));
+
+        Assert.assertFalse(WallUtils.isValidateMySql("select f1, f2 from t union select null, '1', 2", config));
+
+        Assert.assertTrue(WallUtils.isValidateMySql("select f1, f2 from t union select c1, c2", config));
+
+    }
+}