SqlServer中行变列

???? 例如创建一个表

???????????????? create table stz

????????????? (

???????????? stuNO int? primary key,

???????????? subject nvarchar(20) not null,

???????????? score int not null

????????????? )

????????????? insert into stz values(1000,'数学',90)

????????????? insert into stz values(1001,'语文',90)

????????????? insert into stz values(1002,'英语',90)

?

????????????? insert into stz values(1001,90)

????????????? insert into stz values(1000,90)

?

????????????? insert into stz values(1002,90)

--????????????? 现在的情况是

??????????? select * from stz;

????????????????????????????????? ：学号，subject，score

??????????? 我们要把它变成： 学号 ，数学，语文，英语

????????????实现如下：

?????????? select stuNo,

?????????????????????????? 数学=(select sum(score) from stz where subject='数学' and stuNO=shi.stuNo group by stuNO),

????????????????????????? ?语文=(select sum(score) from? stz where subject='语文' and stuNO=shi.stuNO group by stuNo),

?????????????????????????? 英语=(select? sum(score) from stz where subject='英语' and stuno=shi.stuNo group by stuNO)

????????? from stz as shi group by stuNo;

???????????---------------------结束

???????????1，突然想到一些问题，如果我们在多表连接查询时为了性能我们先考虑连接查询，子查询，嵌套查询，知道为什么吗？还有在写查询语句是尽量不要在where 后写运算逻辑？

?????????? 其实这里只有一个共同的原因：嵌套查询，where ，后的条件在你查每条数据时都会运行，很是影响性能。

?????????? 2， 看下这个查语句，有问题吗？select * from (select * from stz);

????????????????? 应改成select * from (select * from stz ) as shi? 必须把那个表集合付给一个变量的。

?????????? 3，看下这段C#代码？会输出什么

?????????????????? int i=2000;

?????????????????? object o=i;

????????????????????i=2001;

??????????????????? int j=(int)o;

????????????????? console.writeLine("{i},{0},{j}");

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