1、背景

由于做的一个小项目里需要联动查询，一想16种情况，因为我是SSM框架写的，这样我就要写16个接口，16个实现，16条sql语句，想想就大头。既然数据库本身接收的就是String类型，那我就直接在implement中写喽，拼接sql语句。

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2、编码内容

1）、mapper.xml中，只需要写一条数据库查询语句即可：

<select id="MapperCommonSelect" resultMap="UserBaseResultMap" parameterType="java.lang.String">
  select 
  u.*,p.*
  from cetc50_user_info u,cetc50_project_info p
  where ${_parameter} u.ProjectId = p.ProjectId
  order by UserId
</select>

${_parameter}就是我要拼接的内容（注意下划线不能少不能少）。

2）、我们再看*impl.java中的文件

  public List<User> selectByProjectIdAndInuseAndApproval(String userName,Integer projectId,Integer inuse,Integer approval){
    List<User> userList = new ArrayList<User>();
    String strsql = "";
    if(userName.equals("")){
        if(projectId > 0){
            if(inuse < 0){
                if(approval < 0){
                    strsql = "u.ProjectId = "+projectId+" and";
                }else{
                    strsql = "u.ProjectId = "+projectId+" and"+" approval = "+approval+" and";
                }
            }else{
                if(approval < 0){
                    strsql = "u.ProjectId = "+projectId+" and"+" inuse = "+inuse+" and";
                }else{
                    strsql = "u.ProjectId = "+projectId+" and"+" approval = "+approval+" and"+" inuse = "+inuse+" and";
                }
            }
        }else{
            if(inuse < 0){
                if(approval < 0){
                    strsql = "";
                }else{
                    strsql = " approval = "+approval+" and";
                }
            }else{
                if(approval < 0){
                    strsql = " inuse = "+inuse+" and";
                }else{
                    strsql = " approval = "+approval+" and"+" inuse = "+inuse+" and";
                }
            }
        }
    }else{
        if(projectId > 0){
            if(inuse < 0){
                if(approval < 0){
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" u.ProjectId = "+projectId+" and";
                }else{
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" u.ProjectId = "+projectId+" and"+" approval = "+approval+" and";
                }
            }else{
                if(approval < 0){
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" u.ProjectId = "+projectId+" and"+" inuse = "+inuse+" and";
                }else{
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" u.ProjectId = "+projectId+" and"+" approval = "+approval+" and"+" inuse = "+inuse+" and";
                }
            }
        }else{
            if(inuse < 0){
                if(approval < 0){
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and";
                }else{
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" approval = "+approval+" and";
                }
            }else{
                if(approval < 0){
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" inuse = "+inuse+" and";
                }else{
                    strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" approval = "+approval+" and"+" inuse = "+inuse+" and";
                }
            }
        }
    }
    
    userList = this.userDao.MapperCommonSelect(strsql);
    return userList;
}

好吧十六种情况，写出来却是有点长。

3、重点问题

说下我遇到的问题吧，这个才是学习的重点。
问题1、但凡写过sql语句的人估计都曾经碰到过类似于Unknown column ‘xxx’ in ‘where clause’的问题。单从字面理解，我们很容易得出列名不存在的结论，但是，很多时候起始并不是由于列名出错造成的。
而是由于拼凑sql语句时对字符类型数据没有用引号引起来造成的。
例如：

strsql = "UserName like CONCAT('%','"+userName+"','%')"+" and"+" inuse = "+inuse+" and";

由于我的userName前面定义的String类型，我一开始没注意，没有增加引号，结果报Unknown column ‘xxx’ in ‘where clause’。

问题2、 报There is no getter for property named ‘strsql’ in ‘class java.lang.String’]错误。

where ${strsql} u.ProjectId = p.ProjectId

我一开始是这样写的，因为strsql是我mapper.java里的传值，请教度娘后，将strsql换成了_parameter，

where ${_parameter} u.ProjectId = p.ProjectId

最后就正确啦。数据也就惊奇的出现在我的前端。开心，哈哈哈