Add get multi level parent method. (#6460)

* Add get multi level parent method.

* Add get multi level parent method.

core/src/main/java/com/alibaba/druid/sql/ast/SQLObject.java

@@ -37,6 +37,7 @@ public interface SQLObject {
     SQLObject clone();
 
     SQLObject getParent();
+    SQLObject getParent(int level);
 
     void setParent(SQLObject parent);
 

core/src/main/java/com/alibaba/druid/sql/ast/SQLObjectImpl.java

@@ -108,6 +108,19 @@ public abstract class SQLObjectImpl implements SQLObject {
     public SQLObject getParent() {
         return parent;
     }
+    public SQLObject getParent(int level) {
+        if (level <= 0) {
+            throw new IllegalArgumentException("Get parent level should be greater than 0.");
+        }
+        SQLObject parent = this;
+        while (level-- > 0) {
+            if (parent == null) {
+                return null;
+            }
+            parent = parent.getParent();
+        }
+        return parent;
+    }
 
     public void setParent(SQLObject parent) {
         this.parent = parent;

core/src/test/java/com/alibaba/druid/bvt/sql/GetParentTest.java

@@ -0,0 +1,21 @@
+package com.alibaba.druid.bvt.sql;
+
+import com.alibaba.druid.sql.ast.SQLObject;
+import com.alibaba.druid.sql.ast.expr.SQLIdentifierExpr;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class GetParentTest {
+    @Test
+    public void test() {
+        SQLObject obj1 = new SQLIdentifierExpr("1");
+        SQLObject obj2 = new SQLIdentifierExpr("2");
+        obj2.setParent(obj1);
+        SQLObject obj3 = new SQLIdentifierExpr("3");
+        obj3.setParent(obj2);
+        Assert.assertEquals(obj2, obj3.getParent(1));
+        Assert.assertEquals(obj1, obj3.getParent(2));
+        Assert.assertNull(obj3.getParent(3));
+        Assert.assertNull(obj3.getParent(4));
+    }
+}