在Mybatis中 @Select注解中如何拼写动态sql

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#mybatis@Select动态拼接SQL #@Select动态拼接SQL #mybatis注解动态拼接SQL

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本文介绍如何在MyBatis Plus中使用@Select注解配合<script>标签实现动态SQL的功能，解决了多表查询时固定SQL带来的不便。

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现在随着mybatis plus的应用，越来越多的弱化了SQL语句，对于单表操作可以说几乎不需要进行自己编写SQL语句了，但对于多表查询操作目前mybatis plus还没有很好的支持，还需要自己编写SQL语句，如：

import java.util.List;

import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import com.baomidou.mybatisplus.mapper.BaseMapper;
import com.shield.base.model.domain.MenuDO;
import com.shield.base.model.param.MenuTreeParam;

/**
 * 基础数据操作对象
 *
 * @author xxx
 * @date 2018/5/18
 */
@Mapper
public interface MenuDAO extends BaseMapper<MenuDO> {

    /**
     * 根据菜单编码获得所有下级菜单列表(包括本级)
     * @param menuId 菜单编码
     * @return 该菜单下的所有菜单列表(包括本级)
     */
    @Select("WITH menuTree"
        + " AS"
        + "("
        + " SELECT menu1.father_rowid as id,menu1.son_rowid as parentId,menu1.system_name as menuName,"
        + "menu1.system_full_rowid as menuTreeFlat,menu1.level_value as menuLevel,menu1.homepage_status as homeStatus,"
        + "menu1.menu_status as menuType,menu1.sort as sort,menu1.duty_name as createName,"
        + "menu1.duty_datetime as createDate,menu1.update_datetime as updateDate,menu1.stop_status as status"
        + " FROM system_menu_setup menu1 WHERE menu1.father_rowid = #{menuId}"
        + " UNION ALL"
        + " SELECT menu2.father_rowid as id,menu2.son_rowid as parentId,menu2.system_name as menuName,"
        + "menu2.system_full_rowid as menuTreeFlat,menu2.level_value as menuLevel,menu2.homepage_status as homeStatus,"
        + "menu2.menu_status as menuType,menu2.sort as sort,menu2.duty_name as createName,"
        + "menu2.duty_datetime as createDate,menu2.update_datetime as updateDate,menu2.stop_status as status"
        + " FROM system_menu_setup menu2"
        + " INNER JOIN menuTree T ON menu2.son_rowid = T.id"
        + ")"
        + " SELECT id,parentId,menuName,MenuTreeFlat,menuLevel,homeStatus,menuType,sort,createName,"
        + "createDate,updateDate,status  FROM menuTree")
    List<MenuDO> selectMenuTreeList(@Param(value = "menuId") Long menuId);

}

这样整个语句基本上都是写死的,没有办法通过参数动态拼接SQL语句，在对于相同语句不同参数来拼接SQL语句是十分不便的,而如果使用xml来配置的话可以用

<where>
	<if test="stopStatus != null">
		and menu.stop_status=#{stopStatus} and roleMenu.stop_status=#{stopStatus}
	</if>
	<if test="menuSource != null">
		and menuSource.menu_source=#{menuSource}
	</if>
	<if test="userId != null">
 		and roleUser.operator_rowid=#{userId}
	</if>
</where>

但是现在很多公司可能会采用@Select注解方式来编写SQL语句，而非通过xml 的SQL Mapper，那对于@Select这种该如何做呢？其实很简单，只是需要用<script>标签包围，然后像xml语法一样书写即可，无须任何其他类或自定义注解类来完成，具体事例如下：

package com.szss.shield.base.dao;

import java.util.List;

import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import com.baomidou.mybatisplus.mapper.BaseMapper;
import com.shield.base.model.domain.MenuDO;
import com.shield.base.model.param.MenuTreeParam;

/**
 * 基础数据操作对象
 *
 * @author xxxx
 * @date 2018/5/18
 */
@Mapper
public interface MenuDAO extends BaseMapper<MenuDO> {   

    /**
     * 根据当前用户权限获取所有权限内的菜单列表（不分页）
     * @param menuTreeParam 菜单参数
     * @return 当前用户权限获取所有权限内的菜单列表
     */
    @Select("<script>"
        + " WITH menuTree"
        + " AS"
        + " ("
        + " SELECT menu.father_rowid as id,menu.son_rowid as parentId,menu.system_name as menuName,\n"
        + " menu.level_value as menuLevel,menu.homepage_status as homeStatus,\n"
        + " menu.menu_status as menuType,menu.sort as sort,menu.stop_status as status,CAST(MAX(menuSource.menu_path_url) as VARCHAR) as menuUrl    \n"
        + "from system_menu_setup menu \n"
        + " LEFT JOIN system_menu_source_setup menuSource\n"
        + " ON menu.father_rowid=menuSource.system_menu_rowid \n"
        + " LEFT JOIN system_role_custom_menu_setup roleMenu \n"
        + " ON menu.father_rowid=roleMenu.system_menu_rowid \n"
        + " LEFT JOIN system_role_operator_setup roleUser \n"
        + " ON roleUser.system_role_setup_rowid=roleMenu.system_role_rowid  \n"
        + " LEFT JOIN system_role_setup role \n"
        + " ON roleUser.system_role_setup_rowid=role.rowid\n"
        + " LEFT JOIN system_department_menu_setup depMenu\n"
        + " ON menu.father_rowid=depMenu.system_menu_rowid\n"
        + "<where>"
        + "<if test='stopStatus != null'>"
        + " and menu.stop_status=#{stopStatus} and roleMenu.stop_status=#{stopStatus}\n"
        +"</if>"
        + "<if test='menuSource != null'>"
        + " and menuSource.menu_source=#{menuSource}"
        +"</if>"
        + "<if test='userId != null'>"
        + " and roleUser.operator_rowid=#{userId}\n"
        +"</if>"
        +"</where>"
        + " GROUP BY menu.father_rowid ,menu.son_rowid ,menu.system_name,menu.level_value,menu.homepage_status,\n"
        + " menu.menu_status,menu.sort,menu.duty_name,menu.duty_datetime,menu.update_datetime,menu.stop_status\n"
        + "  UNION ALL\n"
        + "  SELECT menu2.father_rowid as id,menu2.son_rowid as parentId,menu2.system_name as menuName,\n"
        + " menu2.level_value as menuLevel,menu2.homepage_status as homeStatus,\n"
        + " menu2.menu_status as menuType,menu2.sort as sort,menu2.stop_status as status,CAST('' as VARCHAR) as menuUrl\n"
        + "  FROM system_menu_setup menu2\n"
        + "  INNER JOIN menuTree T ON menu2.father_rowid= T.parentId\n"
        + " )\n"
        + "  SELECT id,parentId,menuName,menuLevel,homeStatus,menuType,sort,status,max(menuUrl) as menuUrl FROM menuTree\n"
        + " GROUP BY id,parentId,menuName,menuLevel,homeStatus,menuType,sort,status\n"
        + " ORDER BY menuLevel,sort"
        + " </script>")
    List<MenuDO> selectMenuTreeListByUserId(MenuTreeParam menuTreeParam);
}

至此我们就可以像在xml文件里面一样愉快的动态拼接你想要的SQL语句了！

注意：在@Select注解中采用<script>标签包围拼接SQL语句时不能在标签里有>大于或<小于符号出现，否则会报Caused by: org.xml.sax.SAXParseException: 元素内容必须由格式正确的字符数据或标记组成。需要对这样的标签符号进行转义即可。