Question

查询1：

SELECT  
                SUM(aol_int) AS AOL,
                SUM(android_phone_int) AS Android_Phone,
                SUM(androidTablet_int) AS Android_Tablet,
                SUM(apple_mail_int) AS Apple_Mail,
                SUM(blackberry_int) AS Blackberry,
                SUM(Eudora_int) AS Eudora,
                SUM(gmail_int) AS Gmail,
                SUM(hotmail_int) AS Hotmail,
                SUM(lotus_notes_int) AS Lotus_Notes,
                SUM(other_int) AS Other,
                SUM(other_webmail_int) as Other_Web_Mail,
                SUM(Outlook_int) AS Outlook,
                SUM(Postbox_int) AS Postbox,
                SUM(sparrow_int) AS Sparrow,
                SUM(thunderbird_int) AS Thunderbird,
                SUM(windowsLiveMail_int) AS Windows_Live_Mail,
                SUM(yahoo_int) AS Yahoo,
                SUM(iPad_int) AS iPad,
                SUM(iPhone_int) AS iPhone,
                SUM(iPod_int) AS iPod

       FROM    mytable;


Query 2: 

select sum(aol_int + android_phone_int + androidtablet_int+apple_mail_int+blackberry_int+Eudora_int+gmail_int+hotmail_int+lotus_notes_int+other_int+other_webmail_int+Outlook_int+Postbox_int+sparrow_int+thunderbird_int+windowsLiveMail_int+yahoo_int+iPad_int+iPhone_int+iPod_int) 
as total_percentage

FROM mytable;

当我总结查询1的结果时，与通过Query2得到的数据相比，我获得的总和不同。 Query2中的值小于查询1.为什么会这样？

故障：

我试着写这样的查询：

SELECT SUM( ISNULL(aol_int,0)           + 
            ISNULL(android_phone_int,0) + 
            ISNULL(androidtablet_int,0) +
            ISNULL(apple_mail_int,0)    + 
            ISNULL(blackberry_int,0)    + 
            ISNULL(Eudora_int,0)        +
            ISNULL(gmail_int,0)         +
            ISNULL(hotmail_int,0)       +
            ISNULL(lotus_notes_int,0)   +
            ISNULL(other_int,0)         +
            ISNULL(other_webmail_int,0) +
            ISNULL(Outlook_int,0)       +
            ISNULL(Postbox_int,0)       +
            ISNULL(sparrow_int,0)       +
            ISNULL(thunderbird_int,0)   +
            ISNULL(windowsLiveMail_int,0)+
            ISNULL(yahoo_int,0)         +
            ISNULL(iPad_int,0)          +
            ISNULL(iPhone_int,0)        +
            ISNULL(iPod_int,0) )AS total_percentage

FROM mytable;

但是，我在MySQL工作台上运行上述查询后出现错误：

错误代码：1582。调用本机函数“ISNULL”时参数计数不正确。这有什么不对？

Answer 1

如果某些行中的某些列包含null，则可能会发生这种情况。发生这种情况时，即使只有一个null列的行中的所有列都会在添加链中产生null，因此该行将不会添加任何内容。

以下是此效果的简短演示。设置：

create table test(x int null, y int null);
insert into test(x,y) values (1,null);
insert into test(x,y) values (null,2);
insert into test(x,y) values (3,3);

查询：

select sum(x+y) from test;     -- Shows 6

select sum(x)+sum(y) from test -- Shows 9

在sqlfiddle上查看此演示：link。

两个查询的结果不同

1 个答案: