常用的 SQL语句------CRUD

复习之前课本上的sql语句，以前上课的时候都是老师在上面讲，我们在下面玩，根本没有把这个放在心上，等到考试的时候临时学习突击下，就可以顺利过60了，但是现在不行了，自己要重新的学习sql，应该把里面最基本的语句和语法都全部的学习下，每个案例都要在数据库中执行下，我们需要这些基本的知识点。

1：创建表，修改表，添加列

/*创建基本表Student*/
create table Student(
           Sno char(5) not null unique,        /*定义的学号不能为空,并且是唯一的值。*/
		   Sname char(20),
		   Ssex char(2),
		   Sage int,
		   Sdept Char(15)
);
/*修改基本表Student*/
     /*添加新列Scome*/
     alter table Student Add Scome DATE;
     /*修改数据类型*/
	 alter table Student MODIFY Sage SMALLINT;
	 /*删除学号的唯一标识*/
	 alter table Student DROP UNIQUE(Son);
/*删除表*/
drop table Student;
/*创建索引*/
    CREATE CLUSTER INDEX Stusname ON Student(Sname);    
/*删除索引*/
    drop index Stusname;

2：基本的查询

/*查询*/
          select Sname,Sage from Student;

	  select Sname,Sage,Sdept from Student;

	  select * from Student;

	  select Sname,1996-Sage from Student;

	  select Sname,'Year of Birth:',1996-Sage,LOWER(Sdept) from Student;

	  select Sname NAME,'Year of Birth:'BIRTH,1996-Sage BIRTHDAY,LOWER(Sdept) DEPARTMENT FROM Student;    /*为列创建别名*/

3：复杂的查询

	    /*消除重复行*/
	     select DISTINCT Sno from Student;

	     /*查询满足条件的元组*/
	     select Sname from Student where Sdept="CD";

		 select Sname,Sage from Student where Sage>=20;

		 alter table Student add Grade int;

		 select DISTINCT Sno from Student where Grade<60;
		 /*确定范围-----------between and */
		 select Sname,Sage,Sdept from Student where Sage between 20 and 23;
		 /*确定集合-----------IN('','','')*/
		 select Sname,Ssex from Student where Sdept IN('IS','MA','CS');
		 /*字符匹配-----------LIKE+++( %：代表任意长度)( _ ：代表单个字符)*/
		 select Sname,Sage from Student where Sname LIKE 'A%';

		 SELECT Sname ,Sage from Student where Sname LIKE 'A_';

		 /*字符匹配-------里面的转义字符-----ESCAPE'\'：表示\为转义字符*/
		/*查询条件是“A_”此时这里的_不是代表一个字符，只是单纯的表示下划线而已。因为语句前面有转义字符。*/
		 select Sname from Student where Sage LIKE 'A\__' ESCAPE '\';

		 /*涉及空值的查询*/
		 select Sname,Sage from Student where Grade IS NULL;  /*查询成绩为空的学生*/

4：更复杂查询

/*多重条件查询*/
   select Sname from Student where Sdept='cd' and Sage>20;

   select Sname from Student where Sdept='cd' or Sage>20;

/*对查询结果进行排序*/

   select Sname Grade from Student where Sage>20 order by Sage DESC;

   select * from Student order by Sage DESC;

/*使用集函数*/
   select count(*) from Student;                 /*求总个数*/

   select count(distinct Sno) from Student;

   select avg(Sage) from Student where Sname='ahui';     /*avg：求平均值*/

   select Sname,count(Sage) from Student group by Sname;

   select MAX(Sage) from Student where Sno='1';    /*最大值*/

5：连接查询

/*连接查询*/  
   /*等值的查询*/
      select Student.*,SC.* from Student,SC where Student.Sno=SC.Sno;

	 /*自然连接两个表*/
	 select Student.Sno,Sname,Ssex,Sage,Sdept,Cno,SC.Grade from Student,SC where Student.Sno=SC.Sno;
	 /*将表Student进行了重新命名，为两个名字，从而进行对自己的查询*/
	 select FIRS.Sage,SECO.Sno from Student FIRS,Student SECO where FIRS.Sno=SECO.Sage; 

	/*外连接*/

	select Student.Sno,Sname,Sage,Sdept,Ssex,Cno,SC.Grade from Student,SC where Student.Sno=SC.Sno(*);

	/*复合条件连接--------就是利用and来进行操作*/

	 select Student.Sno,Sname from Student,SC where Student.Sno=SC.Sno and SC.Cno='2' and Student.Sage=2;

6: 嵌套查询

      这个查询是将最里面的查询结果当作外面查询的条件来进行查询的，执行顺序是从里到外执行的。注意：最里面的查询(子查询的select语句中不能使用ORDER BY子句，ORDER BY子句只能对最终的查询结果进行排序)

   /*嵌套查询---就是把一个查询的结果当做另一个查询的条件来进行查询*/

      select Sname from Student where Sno 
	        IN(
			   select Sno from SC where Cno='2'
			);
			/*--01：带有IN的子查询*/
	  select Sno,Sname,Sdept from Student where Sdept IN(  select Sdept from Student where Sname='ahui');
	       /*--02：带有比较运算符的子查询*/
	  select Sno,Sname,Sdept from Student where Sdept=(  select Sdept from Student where Sname='ahui');

7：集合查询

      每一个select语句都能获得一个或一组元组，若要把多个select语句的结果合并为一个结果，可用集合操作来完成。集合操作主要包括并操作（UNION）交操作（INTERSECT）,差操作（MINUS）.    

      使用UNION将多个查询结果合并起来，形成一个完整的查询结果时，系统会自动去掉重复的元组；

注意的是参加UNION操作的各数据项数目必须相同，对应项的数据类型也要必须相同。

/*集合查询*/
        /*查询计算机系的学生及年龄不大于19学生----是个并集*/
     select * from Student where Sdept='CS' UNION select * from Student where Sage<=19;

	 select Sno from SC where Cno='1' UNION select Sno from SC where Cno='2';

8：数据更新

 /*插入数据*/
  insert into Student values('05','aaa','男','25','3','2015-11-26','3');

  insert into SC(Sno,Cno) values('06','3');

  insert into Deptage(Sdept,Avgage) select Sdept,AVG(Sage) from Student GROUP BY Sdept;  
/*修改数据*/
  
   update Student set Sage=233 where Sno='01';

   update Student Set Sage=Sage+1;

   update SC set Grade=1 where '1'=( select Sdept from Student where Student.Sno=SC.Sno);

/*删除数据*/
   delete from Student where Sno='01';