經過百度,google知道hibernate中hql是不支持union的,所以只能借助native sql了.背景如下:一年前寫了一個hql:

　　原來代碼

　　String countHql2 = "select count

　　(distinct p) from Project as p,CommentSimple as c,ProjectBookmark as b where ("

　　+ "c.owner.id=? and p.id=c.targetId and c.targetType=500) or (b.user.id=? and p.id=b.project.id)";

　　String hql2 = "select distinct p from Project as p,CommentSimple as c,ProjectBookmark as b where ( "+ "c.owner.id=? and p.id=c.targetId and c.targetType=500) or (b.user.id=? and p.id=b.project.id)"; 主要是找出某個人所有評論過或收藏過的項目.簡單表結構如下:

　　project:id owner_id(用戶id)保存項目的基本信息

　　project_bookmark:uid(用戶id),project_id(收藏的項目的id),owner_id(收藏者的id)

　　comment_simple:target_type(保存對某種對象的評論,值為500時表示的對項目的評論),target_id(保存對某種對象的評論,值為該對象的id),project_id(項目的id),owner_id(評論者的id)

　　由于這個sql執行時所建的索引無法使用,而且還造成了三個表連接會有大量的無效的查詢以及重復結果,最后還得要distinct可以想象執行的效率.

　　只好改用union來重寫,需要用到hibernate的native sql,經過努力終于找到可以用union找出整個對象以及在配置文件中與該對象有關系的對象的方法.

　　與其說是找出來的,不如說是試出來的,代碼如下:

　　union

　　String sql1 = "SELECT COUNT(*) FROM(SELECT p.id FROM project p,comment_simple c WHERE p.id=c.target_id AND c.target_type=500 AND c.uid=" + userId + " UNION SELECT pr.id FROM project pr,project_bookmark b WHERE pr.id=b.project_id AND b.uid=" + userId + ") AS temp"; String sql2 = "(SELECT {p.*} FROM project p,comment_simple c WHERE p.id=c.target_id AND c.target_type=500 AND c.uid=" + userId + ")" + "UNION" + "(SELECT {p.*} FROM project p,project_bookmark b WHERE p.id=b.project_id AND b.uid=" + userId + ")LIMIT " + (pageIndex - 1) * maxPerPage + "," + maxPerPage; SQLQuery query = this.getSession().createSQLQuery(sql1); Integercount=Integer.valueOf(((BigInteger)query.uniqueResult()).toString()); SQLQuery query2 = this.getSession().createSQLQuery(sql2); query2.addEntity("p", Project.class); List<Project> list = query2.list();

　　sql1符合條件的項目的總數.sql2求出符合條件項目的某一頁.

　　要注意的是:sql2中{p.*}要寫成一樣的.

　　簡而言之:select {a.*} from A a where ... union select {a.*} from A a where...

　　如果還要排序的話sql2換成sql3:

　　需要order by時

　　String sql3 = "(SELECT {p.*},p.created FROM project_hz p,comment_simple c WHERE p.id=c.target_id AND c.target_type=500 AND c.uid=" + userId + ")" + "UNION" + "(SELECT {p.*} ,p.created FROM project_hz p,project_bookmark b WHERE p.id=b.project_id AND b.uid=" + userId + ") ORDER BY created LIMIT " + (pageIndex - 1) * maxPerPage + "," + maxPerPage; 要注意的是p.created(需要排序的那個字段) 要個別標出,因為hibernate在轉換為sql是會寫成 select created as ...所以排序時將不起作用,需要我們自己標出.

　　這里只是找出了一個解決方案,因為對hibernate的調優不是特別熟悉這樣做會不會造成另外不好的影響就不清楚了,如果有高手看到有什么問題,或者有更好的方法.請不吝賜教!!