MySQL的IFNULL()、ISNULL()和NULLIF()函数实例讲解
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2022-03-09 22:10:20
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...
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
如果expr1= expr2 成立,那么返回值为NULL,否则返回值为expr1。这和CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END相同。
mysql> SELECT NULLIF(1,1); -> NULL mysql> SELECT NULLIF(1,2); -> 1
如果参数不相等,则 MySQL 两次求得的值为 expr1。
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