bugfix:union子查询bug

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

@@ -0,0 +1,50 @@
+package com.alibaba.druid.sql.ast.statement;
+
+import com.alibaba.druid.sql.visitor.SQLASTVisitor;
+
+public class SQLUnionQueryTableSource extends SQLTableSourceImpl {
+
+    private SQLUnionQuery union;
+
+    public SQLUnionQueryTableSource(){
+
+    }
+
+    public SQLUnionQueryTableSource(String alias){
+        super(alias);
+    }
+
+    public SQLUnionQueryTableSource(SQLUnionQuery union, String alias){
+        super(alias);
+        this.setUnion(union);
+    }
+
+    public SQLUnionQueryTableSource(SQLUnionQuery union){
+        this.setUnion(union);
+    }
+
+    @Override
+    protected void accept0(SQLASTVisitor visitor) {
+        if (visitor.visit(this)) {
+            acceptChild(visitor, union);
+        }
+        visitor.endVisit(this);
+    }
+
+    public void output(StringBuffer buf) {
+        buf.append("(");
+        this.union.output(buf);
+        buf.append(")");
+    }
+
+    public SQLUnionQuery getUnion() {
+        return union;
+    }
+
+    public void setUnion(SQLUnionQuery union) {
+        if (union != null) {
+            union.setParent(this);
+        }
+        this.union = union;
+    }
+}