在使用left join等联表查询时，常遇到某些字段为null，一般都在后台语言使用if (a==null) 判断做处理，其实MySQL本身也有一个IFNULL函数可以处理。同时我们也来说说ISNULL()和NULLIF()函数。

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一、IFNULL(expr1,expr2)用法

假如expr1不为NULL，则 IFNULL() 的返回值为expr1;否则其返回值为 expr2。IFNULL()的返回值是数字或是字符串，具体情况取决于其所使用的语境。

      mysql>   SELECT   IFNULL(1,0);   
                            ->   1   
      mysql>   SELECT   IFNULL(NULL,10);   
                            ->   10   
      mysql>   SELECT   IFNULL(1/0,10);   
                            ->   10   
      mysql>   SELECT   IFNULL(1/0,'yes');   
                            ->   'yes'  

IFNULL(expr1,expr2)的默认结果值为两个表达式中更加“通用”的一个，顺序为STRING、REAL或 INTEGER。

项目中具体代码

  public static List<ta_activity_info> ChooseByActivityID(string ActivityID)  
      {  
          string strSql = @"SELECT DISTINCT  
    @ActivityID as ActivityID,  
   IFNULL(ta_choujiang.CreateTime,(SELECT ta_activity_info.CreateTime from ta_activity_info where ta_activity_info.ActivityID=@ActivityID))as CreateTime,  
    (  
        SELECT  
            count(*)  
        FROM  
            ta_choujiang  
        WHERE  
            ta_choujiang.ActivityID = @ActivityID  
    ) AS PeopleSum,  
    (  
        SELECT  
            count(*)  
        FROM  
            ta_wininfo  
        WHERE  
            ta_wininfo.ActivityID = @ActivityID  
    ) AS zhongjiangPeople,  
    (  
        SELECT  
            (  
                (  
                    SELECT  
                        count(*)  
                    FROM  
                        ta_choujiang  
                    WHERE  
                        ta_choujiang.ActivityID = @ActivityID  
                ) - (  
                    SELECT  
                        count(*)  
                    FROM  
                        ta_wininfo  
                    WHERE  
                        ta_wininfo.ActivityID =@ActivityID  
                )  
            )  
    ) AS weizhongjiangPeople  
FROM  
    ta_activity_info  
LEFT JOIN ta_choujiang ON ta_activity_info.ActivityID = ta_activity_info.ActivityID  
and ta_choujiang.ActivityID =@ActivityID";  
  
          //参数赋值  
          MySqlParameter[] param =  
                       {  
                          new MySqlParameter("@ActivityID", MySqlDbType.VarChar)  
                       };  
          param[0].Value = ActivityID.ToString();  
          //return GetList2<ta_rewardinfo>(strSql, param);  
          List<ta_activity_info> List = GetListV3<ta_activity_info>(strSql, param);  
          return List;  
      }  

  public static List<ta_activity_info> ChooseByActivityID(string ActivityID)  
      {  
          string strSql = @"SELECT DISTINCT  
    @ActivityID as ActivityID,  
   IFNULL(ta_choujiang.CreateTime,(SELECT ta_activity_info.CreateTime from ta_activity_info where ta_activity_info.ActivityID=@ActivityID))as CreateTime,  
    (  
        SELECT  
            count(*)  
        FROM  
            ta_choujiang  
        WHERE  
            ta_choujiang.ActivityID = @ActivityID  
    ) AS PeopleSum,  
    (  
        SELECT  
            count(*)  
        FROM  
            ta_wininfo  
        WHERE  
            ta_wininfo.ActivityID = @ActivityID  
    ) AS zhongjiangPeople,  
    (  
        SELECT  
            (  
                (  
                    SELECT  
                        count(*)  
                    FROM  
                        ta_choujiang  
                    WHERE  
                        ta_choujiang.ActivityID = @ActivityID  
                ) - (  
                    SELECT  
                        count(*)  
                    FROM  
                        ta_wininfo  
                    WHERE  
                        ta_wininfo.ActivityID =@ActivityID  
                )  
            )  
    ) AS weizhongjiangPeople  
FROM  
    ta_activity_info  
LEFT JOIN ta_choujiang ON ta_activity_info.ActivityID = ta_activity_info.ActivityID  
and ta_choujiang.ActivityID =@ActivityID";  
  
          //参数赋值  
          MySqlParameter[] param =  
                       {  
                          new MySqlParameter("@ActivityID", MySqlDbType.VarChar)  
                       };  
          param[0].Value = ActivityID.ToString();  
          //return GetList2<ta_rewardinfo>(strSql, param);  
          List<ta_activity_info> List = GetListV3<ta_activity_info>(strSql, param);  
          return List;  
      }  

二、ISNULL(expr) 的用法

如expr 为null，那么isnull() 的返回值为 1，否则返回值为 0。

    mysql> select isnull(1+1);
                       -> 0
    mysql> select isnull(1/0);
                       -> 1

使用= 的null 值对比通常是错误的。

三、NULLIF(expr1,expr2)用法