mybatis plus中的${ew.sqlSegment}，${ew.sqlSelect}，${ew.customSqlSegment}使用

ew是mapper方法里的@Param(Constants.WRAPPER) Wrapper queryWrapper对象

首先判断ew.emptyOfWhere是否存在where条件，有的话再拼接上去，ew.customSqlSegment是WHERE + sql语句
没有where的时候加上 == false

使用${ew.sqlSegment} 如果是连表查询且查询条件是连表的字段则需在service层拼接查询条件时字段前指定别名

最佳例子1

mapper.xml

 <select id="tableList" resultType="java.util.LinkedHashMap">
        SELECT
            ${ew.sqlSelect} // 这里拼接select后面的语句
        FROM
            ${table_name} //如果是单表的话，这里可以写死
        ${ew.customSqlSegment}
    select>

mapper

IPage<LinkedHashMap<String,Object>> tableList(@Param("table_name") String table_name,
										Page page,
                                           @Param(Constants.WRAPPER) QueryWrapper queryWrapper);

test

String responseField = "*"; 
queryWrapper.select(responseField);
// 即 select * ...

String responseField = "name";
queryWrapper.select(responseField);
// 即 select name ...

最佳例子2

controller

 public String saveAddress(HttpSession session) {
        UserVO user1 = (UserVO)session.getAttribute("user");

        LambdaQueryWrapper<User> lambdaQueryWrapper = Wrappers.<User>lambdaQuery()
                .select(User::getNickName, User::getUserId) // 需要查询的列，即 ${ew.sqlSelect}
                .eq(User::getUserId, user1.getUserId());// 条件
        User user = this.userMapper.selectNickNameAndUserId(lambdaQueryWrapper);
        System.out.println(user);
        return null;
    }

mapper

    User selectNickNameAndUserId(@Param(Constants.WRAPPER) LambdaQueryWrapper<User> queryWrapper);

mapper.xml

<select id="selectNickNameAndUserId" resultType="com.example.demo.entity.User">
        select
            <if test="ew != null and ew.SqlSelect != null and ew.SqlSelect != ''">
                ${ew.SqlSelect}
            if>
        from
            user
        where is_deleted != 1
        <if test="ew != null">
            <if test="ew.nonEmptyOfWhere">
                AND
            if>
            ${ew.sqlSegment}
        if>
    select>

 <select id="selectNickNameAndUserId" resultType="com.example.demo.entity.User">
        select
            <if test="ew != null and ew.SqlSelect != null and ew.SqlSelect != ''">
                ${ew.SqlSelect}
            if>
        from
            user
        ${ew.customSqlSegment}
    select>

使用${ew.sqlSegment} 如果是联表查询且查询条件是连表的字段则需在service层拼接查询条件时字段前指定别名，而且不能用lambda的查询了

 <select id="selectByRoleId" resultType="com.captain.crewer.mybatis.plus.dto.RolePermsDTO">
        SELECT tp.id,
               tp.perm_name,
               tp.url,
               tr.role_id   as roleId,
               tr.role_name as roleName
        FROM tb_role tr
                     LEFT JOIN tb_perm_role tpr ON tr.role_id = tpr.role_id
                     LEFT JOIN tb_perm tp ON tpr.perm_id = tp.id ${ew.customSqlSegment}
    select>

mapper

 List<RolePermsDTO> selectByRoleId(@Param(Constants.WRAPPER) Wrapper<RolePermsDTO> wrapper);

 @Test
    public void test2(){
        QueryWrapper<RolePermsDTO> wrapper = new QueryWrapper<>();
        wrapper.eq("tr.role_id", 1);
        
        tbPermService.selectByRoleId(wrapper);
    }