三十二、MyBatis註解SQL
指令碼sql
XML配置方式的動態SQL我就不講了,有興趣可以自己瞭解,下面是用<script>的方式把它照搬過來,用註解來實現。適用於xml配置轉換到註解配置很明顯,在java中寫xml可讀性和維護性太差,尤其當SQL很長時,這樣寫是很痛苦的。
//查詢所有,@Results欄位裡只需對映實體類和表字段名不一致的欄位名 @Select(" SELECT * FROM child_channel ORDER BY rank ASC,online_status DESC") @Results({ @Result(property = "onlineStatus", column = "online_status"), @Result(property = "hasNew", column = "has_new"), @Result(property = "updateTm",column = "update_tm"), @Result(property = "createTm",column = "create_tm") }) List<Channel> findChannelList(); //用函式 @Select("<script>" + " SELECT COUNT(*) FROM child_unit WHERE 1=1 " + " <when test='params.category != null'> AND category = #{params.category} </when> " + " <when test='params.onlineStatus != null'> AND online_status = #{params.onlineStatus} </when> " + " ORDER BY id ASC" + "</script>") Integer count(@Param(value = "params") Map<String, Object> params); //連線查 @Select(" SELECT * FROM child_unit c1 JOIN child_unit_position c2 ON c2.unit_id = c1.id WHERE c2.unit_group_id = #{unitGroupId} ORDER BY c2.rank ASC") @Results({ @Result(property = "subTitle", column = "sub_title"), @Result(property = "contentId", column = "content_id"), @Result(property = "openType", column = "open_type"), @Result(property = "onlineStatus",column = "online_status"), @Result(property = "hasNew",column = "has_new"), @Result(property = "createTm", column = "create_tm"), @Result(property = "updateTm", column = "update_tm") }) //刪除 @Delete("<script>" + " DELETE FROM child_channel WHERE id IN " + " <foreach item='item' index='index' collection='idSet' open='(' separator=',' close=')'>" + " #{item}" + " </foreach>" + "</script>") int deleteByPrimaryKeySet(@Param("idSet") Set<Long> idSet);
在方法中構建sql
dao介面中是不能寫實現的,所以這裡借用內部類來生成動態SQL。增改刪也有對應的@InsertProvider、@UpdateProvider、@DeleteProvider
@Mapper
public interface MybatisDao {
//使用UserDaoProvider類的findUserById方法來生成sql
@SelectProvider(type = UserDaoProvider.class, method = "findUserById")
public List<User> findUserById(User user);
class UserDaoProvider {
public String findUserById(User user) {
String sql = "SELECT * FROM user";
if(user.getId()!=null){
sql += " where id = #{id}";
}
return sql;
}
}
這比<script>更加清晰,適用於查詢語句不是很長、條件不多的場景,SQL很直觀。但是在寫很長的SQL時,這樣拼接SQL同樣會很痛苦
結構化SQL
public String findUserById(User user) { return new SQL(){{ SELECT("id,name"); SELECT("other"); FROM("user"); if(user.getId()!=null){ WHERE("id = #{id}"); } if(user.getName()!=null){ WHERE("name = #{name}"); } //從這個toString可以看出,其內部使用高效的StringBuilder實現SQL拼接 }}.toString(); }
這是把前面的內部類改造一下
SELECT:表示要查詢的欄位,如果一行寫不完,可以在第二行再寫一個SELECT,這兩個SELECT會智慧的進行合併而不會重複
FROM和WHERE:跟SELECT一樣,可以寫多個引數,也可以在多行重複使用,最終會智慧合併而不會報錯
這樣語句適用於寫很長的SQL時,能夠保證SQL結構清楚。便於維護,可讀性高。但是這種自動生成的SQL和HIBERNATE一樣,在實現一些複雜語句的SQL時會束手無策。所以需要根據現實場景,來考慮使用哪一種動態SQL
上面的例子只是最基本的用法:更多詳細用法,可以參考mybatis中文網的專門介紹
http://www.mybatis.org/mybatis-3/zh/statement-builders.html
List傳值錯誤
動態SQL中,有時要對批量資料進行處理,難免會使用list做為引數
@SelectProvider(type = UserDaoProvider.class, method = "find") public List<Map> find(List list); class UserDaoProvider { public String find(List list) {
這是一個最簡單的list傳參,但是在執行時會報傳參錯誤。這是mybatis內部機制造成的,其引數需要是key/value結構,當遇到這裡不是key/value結構的list時,mybatis會自己把它轉換成key/value結構,key就是他的名字"list",value就是他的值List,要正確傳參需要使用key/value結構的map,如下
@SelectProvider(type = UserDaoProvider.class, method = "find") public List<Map> find(List list); class UserDaoProvider { public String find(Map map) { List list = (List) map.get("list");