+-Many multi-table query three tables even check the sub-queries +

Multi-table query 
    query multiple tables of data needed 
    for instance: there are classes and students table table 
        you have to consult the class name data for all participants 
        will check the table to get the class id a class and then go to college based on the id corresponding table query student 

    prepare data: 
    Create Table EMP (int ID, name char (10), Sex char, the dept_id int); 
     INSERT EMP values (. 1, "rhubarb", "m",. 1); 
     INSERT EMP values (2, "Wang "," m ", 2); 
     INSERT EMP values (. 3," talk of the town "," W ", 30); 

    Create Table Dept (ID int, char name (10)); 
    INSERT Dept values (. 1," market " ); 
    INSERT the dept values (2, "financial"); 
    INSERT the dept values (3, "administrative"); 

    the way multi-table queries 
    1. Cartesian product queries 
        what is Cartesian product,With a record coordinates to link all the records in another table 
        like the two tables of data to do a multiplication 
        which will result in a lot of useless duplication of data 
            we have to the effect that: employee id and department table the department table spliced together on the same id
        Screening with the correct data where 
      the SELECT * from emp, the dept where emp.dept_id = dept.id; 

      ON key 
        role in a multi-table query conditions is 
        select * from emp, dept on emp.dept_id = dept.id; this is because on the wrong syntax can be used only in special multi-table queries in 


      connection queries within 2. 
        inner the Join 
        the SELECT * from emp inner emp.dept_id on the Join the dept = dept.id; 

       # query for all employees and they belong Segment information 
      3. left outer join 
        left join 
       left of the data tables is an exact match the data displayed on the right side of the table to display only 
       the SELECT * from emp left join the dept oN emp.dept_id = dept.id; 

       # to query all sectors and all their employee information 
      4. right outer join 
        right join 
        the table data matches the left to the right until the display data in the table show complete 
        select * from emp right join dept on emp.dept_id = dept.id;

       # Displays all the data in a plurality of tables Table 
      5. Full linked 
        full join mysql oracle support does not support 
        can be achieved indirectly by Union 
                        Union represents a combined query to a plurality of means merge query results show 
                        requirement is merged table structure must be the same 
                        default deduplication 

                        combined repeated but without removing the 
                        Union All 

       SELECT * from the Join Dept EMP right emp.dept_id ON = dept.id 
       Union 
       SELECT * from the Join Dept EMP left ON emp.dept_id = dept.id; 




      summary: multi-table Fill in the blanks in writing in accordance with the link to the left if you want to display all written with a left join 
            all right join with the right 
            to display all the results and consolidated results of left and right links links 
            course, you can check together more tables but does not make sense and you should try avoid too many tables to check with 
            a maximum of three in a many-time

    select *from emp left join dept left join xxtable on emp.dept_id = dept.id;

create table tec(id int,name char(10));
insert into tec value(1,"egon");
insert into tec value(2,"yyh");

create table stu(id int,name char(10));
insert into stu value(1,"大傻");
insert into stu value(2,"中傻");
insert into stu value(3,"小傻");
create table s_t(s_id int,t_id int);
insert into s_t value(1,2);
insert into s_t value(2,2);
insert into s_t value(3,1);

需求  找出 yyh 这个老师 教过的学生信息
思路：
    第一步 到关系表中 去查询 哪些老师教过哪些学生（学生的id）  形成了一个临时表
    第二步  将上一步得到临时表 与 学生表进行连接
    第三步 加上额外的筛选条件   老师的name  是  yyh


    select tec.name teacher,stu.name student from
    tec inner join s_t on tec.id = s_t.t_id
    inner join stu on s_t.s_id = stu.id
    where tec.name = "egon" ;

子查询
    什么是子查询：将上一次查询的结果 作为本次查询的原始数据（或是查询条件）
    也就是嵌套查询  这中嵌套的语法一班不让写,不允许使用,