SQL Server 2012 - SQL查询

1. 执行计划显示SQL执行的开销
2. 
3. 工具→ SQL Server Profiler : SQL Server 分析器，监视系统调用的SQL Server查询
4. Top查询

   -- Top Percent 选择百分比
   SELECT top 30 percent *  FROM [SchoolDB].[dbo].[Student]
   
   -- Top 支持定义变量查询
   declare @per int = 30
   SELECT top ( @per) percent *  FROM [SchoolDB].[dbo].[Student]　

5.  
6. 
7. 常用Where查询

   -- Top Percent 选择百分比
   SELECT top 30 percent *  FROM [SchoolDB].[dbo].[Student]
   
   -- Top 支持定义变量查询
   declare @per int = 30
   SELECT top ( @per) percent *  FROM [SchoolDB].[dbo].[Student]
   
   --重命名
   select StuName as [姓 名]  FROM [SchoolDB].[dbo].[Student]
   
   --通配符查询
   select *  FROM [SchoolDB].[dbo].[Student] where  Height like '1[67][0-9]'
   
   --IsNull 把Null数据转换为其他值。NULL和任何字段做比较返回的都是NULL
   select *  FROM [SchoolDB].[dbo].[Student] where  ISNULL(StuBirthday,0)  <'1990-01-01'
   
   -- Exists 存在，适用于内外表的查询，Exists不关心查询到的内容，只关心是否能查询到数据
   select *　From [Student] as Stu1 where exists(
   	 --select 1 
   	 select * From [Student] as Stu2 where StuID=Stu1.StuID  and Stu2.Height>160
   )
    
   
   --CharInedx 字符a在字符串aa中出现的位置
   select CHARINDEX('a','sdafasdad',1)
   
   --PatIndex ,查询通配符字符串的位置
   select * From [Student] where PatIndex ('王_',StuName)>0

8. 聚合函数

   -------------聚合函数-------------
   
   --group by 单字段分组-- 
   
   --每个班级的人数
   select class as '班级', count(*) as '人数' from [Student] 
   group by Class
   
   --每个班级的平均身高、最小生日
   select class as '班级',AVG(Height) as '平均身高',Min(StuBirthday) as '最小生日'  from [Student] 
   group by Class
   
   select class as '班级',Min(StuBirthday) as '平均身高' from [Student] 
   group by Class
   
   --group by 多字段分组-- 
   select class as '班级',StuSex as '性别',COUNT(*),AVG(Height) as '平均身高',Min(StuBirthday) as '最小生日'  from [Student] 
   group by Class,StuSex
   -- having Class='1'  --分组后进行过滤
   having  AVG(Height)>=159  --分组后进行过滤
   
   --SUM求和，AVG平均，MAX最大，Cast转化
   select class as '班级', SUM(height) as '总身高',MAX(height) as '身高最高', CAST(AVG(height/1.0) as decimal(18,2))  as '平均高'  from [Student] 
   group by Class
   
   --Distinct
   select class, count(*) as '班级人数',count(1) as '班级人数',count(StuID) as '班级人数',count(StuSex) as '班级性别',count(distinct(StuSex)) as '班级不同的性别'
   from [Student] 
   group by Class

9. 嵌套查询

   ------------嵌套查询----------
    
   ------------1.1子查询 ----------------
   SELECT  *
   FROM    ( SELECT    dbo.Student.* ,
                       dbo.ClassInfo.ClassName
             FROM      dbo.Student
                       JOIN dbo.ClassInfo ON Student.Class = dbo.ClassInfo.ID
           ) AS T;
   -- T 是临时的查询结果集
   
   ------------1.1 嵌套子查询（内部子查询结果一次性提供结果集供外部查询） ----------------
   SELECT  *
   FROM    dbo.Student
   WHERE   Class IN ( SELECT   ID
                      FROM     dbo.ClassInfo );
   
    ------------1.2 相关子查询（外部表中的数据逐条作为参数传递给内部表中的数据） ----------------
   SELECT  *
   FROM    dbo.Student AS Stu
   WHERE  EXISTS ( SELECT * FROM dbo.ClassInfo  WHERE ID=Stu.Class  );
   
   -- IN :子查询数据量小，而外表数据大 (子查询数据逐条循环)
   -- Exist:子查询数据量大，而外表数据小 （Exist只关心是否能查询到数据）
   -- 能用关联查询，尽量不用子查询
   
   
   -----------2. 创建物理临时表 -----------
   CREATE TABLE #TableTemp
       (
         id INT NOT NULL
                IDENTITY(1, 1)
                PRIMARY KEY ,
         name NVARCHAR(10)
       );
   
   INSERT  INTO #TableTemp
           ( name )
   VALUES  ( N'你好'  -- name - nvarchar(10)
             );
   
   SELECT  *
   FROM    #TableTemp;
   
   DROP TABLE #TableTemp;
   
   -----------3. into 临时表(物理表)---------
   SELECT  dbo.Student.* ,
           dbo.ClassInfo.ClassName
   INTO    #TableTemp
   FROM    dbo.Student
           JOIN dbo.ClassInfo ON Student.Class = dbo.ClassInfo.ID;
   
   SELECT  *
   FROM    #TableTemp;
   
   DROP TABLE #TableTemp;
   
   ----------4.复制表结构----------
   SELECT  *
   INTO    Student_His
   FROM    dbo.Student
   WHERE   1 = 2;
   
   ----------5.创建表变量----------
   DECLARE @tmp_Table TABLE
       (
         id INT NOT NULL
                IDENTITY(1, 1)
                PRIMARY KEY ,
         name NVARCHAR(20)
       );
   
   INSERT  @tmp_Table
           ( name )
   VALUES  ( -- id - int
             N'表变量'  -- name - nvarchar(20)
             );
   
   SELECT  *
   FROM    @tmp_Table;
   
   ----------6.公共表表达式， CTE:Common Table Expression。可以生成多个临时表，然后再进行连接查询---------- 
   ;WITH  T AS ( SELECT   dbo.Student.* ,
                           dbo.ClassInfo.ClassName
                  FROM     dbo.Student
                           JOIN dbo.ClassInfo ON Student.Class = dbo.ClassInfo.ID
                ),
   			 T1 AS(
   				SELECT * FROM dbo.Student
   			 )
   	-- Select、Update 紧跟在 With As 才有效		 
       SELECT  T.*
       FROM    T JOIN T1 ON t.StuID=T1.StuID 
   
   ----------7.使用ANY、SOME、ALL关键字----------
   SELECT  *
   FROM    dbo.Student
   WHERE   Class =ALL ( SELECT   ID
                      FROM     dbo.ClassInfo  WHERE ID=1);