學步園

select * from score create database demo use demo create table [user] ( [uId] int identity( 1 ,1 ) primary key, [name] varchar ( 50), [level] int --1骨灰大蝦菜鳥 ) insert into [user] ( name, level ) values (' 犀利哥 ', 1 ) insert into [user] ( name, level ) values (' 小月月 ', 2 ) insert into [user] ( name, level ) values (' 芙蓉姐姐 ', 3 ) --case end 單值判斷 相當於 switch case --then 後面的返回值類型必須一致 select [name] , case [level] when 1 then '骨灰 ' when 2 then '大蝦 ' when 3 then '菜鳥 ' end as '等級 ' from [user] use MySchool select * from score --case end 第二種用法，相當於多重 if 語句 select studentId , case when english >=90 then ' 優 ' when english >=80 and english <90 then ' 良 ' when english >=70 and english < 80 then ' 中 ' when english >= 60 and english < 70 then ' 可 ' else ' 差 ' end as '成績 ' from score order by english -- 表中有A B C 三列 ,用 SQL 語句實現：當 A列大於 B 列時選擇A 列否則選擇 B 列，當B 列大於 C列時選擇 B 列否則選擇 C列。 select case when a > b then a else b end , case when b > c then b else c end from T -- 練習 create table test ( number varchar ( 10), amount int ) insert into test( number ,amount ) values ( 'RK1', 10 ) insert into test( number ,amount ) values ( 'RK2', 20 ) insert into test( number ,amount ) values ( 'RK3',- 30 ) insert into test( number ,amount ) values ( 'RK4',- 10 ) select number , case when amount > 0 then amount else 0 end as '收入 ' , case when amount < 0 then abs ( amount) else 0 end as '支出 ' from test --結果如下

-- 有一張表student0 ，記錄學生成績 use demo CREATE TABLE student0 ( name nvarchar (10 ), subject nvarchar (10 ), result int ) INSERT INTO student0 VALUES (' 張三 ', ' 語文' , 80) INSERT INTO student0 VALUES (' 張三 ', ' 數學' , 90) INSERT INTO student0 VALUES (' 張三 ', ' 物理' , 85) INSERT INTO student0 VALUES (' 李四 ', ' 語文' , 85) INSERT INTO student0 VALUES (' 李四 ', ' 數學' , 92) INSERT INTO student0 VALUES (' 李四 ', ' 物理' ,null) select * from student0 select [name] , isnull (sum ( case subject when ' 語文 ' then result end ),0 ) as '語文 ' , isnull (sum ( case subject when ' 數學 ' then result end ),0 ) as '數學 ' , isnull (sum ( case subject when ' 物理 ' then result end ),0 ) as '物理 ' from student0 group by [name]

-- 子查詢將一個查詢語句做為一個結果集供其他 SQL 語句使用，就像使用普通的表一樣， -- 被當作結果集的查詢語句被稱為子查詢。所有可以使用表的地方几乎都可以使用子查詢來代替。 use myschool select sName from ( select * from student ) as t select 1,( select sum ( english) from score ) as ' 和 ',( select avg ( sAge) from student ) as ' 平均年齡 ' -- 查詢高一一班所有的學生 select * from student where sClassId = ( select cId from class where cName = '高一一班 ' ) -- 查詢高一一班 高二一班所有的學生 -- 子查詢返回的值不止一個。當子查詢跟隨在 = 、!= 、 <、 <= 、> 、 >= 之後 -- 子查詢跟在比較運算符之後，要求子查詢只返回一個值 -- 如果子查詢是多行單列的子查詢，這樣的子查詢的結果集其實是一個集合。可以使用 in 關鍵字代替 =號 select * from student where sClassId = ( select cId from class where cName in ( '高一一班 ' ,' 高二一班 ')) select * from student where sClassId in ( select cId from class where cName in ( '高一一班 ' ,' 高二一班 ')) -- 查詢劉關張的成績 select * from score where studentId in ( select sId from student where sName in ( '劉備 ' ,' 關羽 ', ' 張飛' )) -- 刪除劉關張 delete from score where studentId in ( select sId from student where sName in ( '劉備 ' ,' 關羽 ', ' 張飛' )) -- 實現分頁 -- 最近入學的個學生 select top 3 * from student order by sId desc -- 查詢第到個學生 select top 3 * from student where sId not in ( select top 3 sId from student order by sId desc) order by sId desc -- 查詢到的學生 select top 3 * from student where sId not in ( select top 6 sId from student order by sId desc) order by sId desc -- 上面是sql 2000 以前的實現方式。 SQLServer2005 後增加了Row_Number 函數簡化實現。 --sql 2005 中的分頁 select * from ( select row_number () over (order by sId desc ) as num,* from student ) as t where num between 1 and 3 select * from ( select row_number () over (order by sId desc ) as num,* from student ) as t where num between 4 and 6 select * from ( select row_number () over (order by sId desc ) as num,* from student ) as t where num between 7 and 9 select * from ( select row_number () over (order by sId desc ) as num,* from student ) as t where num between 3 *( 3- 1 ) + 1 and 3 *3 -- 表連接 -- 交叉連接cross join select * from student cross join class -- 內連接inner join...on... select * from student inner join class on sClassId = cId select * from class -- 查詢所有學生的姓名、年齡及所在班級 select sName , sAge, cName ,sSex from student inner join class on sClassId = cId where sSex =' 女 ' -- 查詢年齡超過歲的學生的姓名、年齡及所在班級 select sName , sAge, cName from class inner join student on sClassId = cId where sAge > 20 -- 外連接 --left join...on... select sName , sAge, cName from class