java learning course development database

A multi-table query the database

1. classification

The combined result set union union all

Join query

En

Outer join

Left outside

Right outside

Natural connection

Subqueries

1. The combined result set, the query result is two select statements merged together

Requires two results are combined, and the number of columns must be the same type of column

select * from a union select id,name from b;

select * from a union all select id,name from b;

1. Join query (***)

Join query is to compute the product of a plurality of tables, a and b tables, the query result is connected to a * b

select * from stuj,score;

Primary foreign key relationships may be used to remove unnecessary information equation

Join query will produce a Cartesian product

select * from stuj,score where stuj.id=score.id;

1. Internal link: link within the above statement is the statement, but not the standard way

sql standards in the link:

select * from stuj inner join score on stuj.id=score.id;

Features: Query result must satisfy the condition of Equation

1. External link

The possibility of query results may occur conditions are not satisfied: Features

Left outside: to the left of the main table table

select * from stuj as s left outer join score c on s.id = c.id;

select * from score as s left outer join stuj c on s.id = c.id;

Outer right: the right of the main table to table

1. Natural connection

Not need to give the primary foreign key relationship equation, it will automatically find the

select * from score natural join stuj;

1. Subquery (****)

A select statement contains another select statement

Also called nested query

Features:

  According to the position of sub-query:

After where: Conditions

After from: to do list

When a subquery appears in the where, you can use the following keyword

      any

   all

The results in the form of sub-queries

      Single separate: Conditions

   Single line multi-column: Conditions

   Multi-line single: Conditions

Rows and columns: Table

- Query paid more than the person's information xiaosan

- Query xiaosan wages

select salary from emp where name=’xiaosan’;

select * from emp where salary>( select salary from emp where name=’xiaosan’);

- Query paid more than 1 greater than the number of people of all the information department

No. --1 largest sector wages

select max(salary) from emp where deptid=1;

- Query No. 1002 of the person's name, salary, department and department address

select name,salary,deptid from emp e where id=1002;

select e.name,e.salary,e.deptid,d.address from emp e,dept d

where e.deptid = d.deptid and id=1002;

select e.name,e.salary,e.deptid,d.address from (select id,salary,deptid from emp) e,dept d where e.deptid = d.deptid and id=1002;

 

1. Connect to their own self-connection

- Query No. 1004 of the names and numbers leadership and leadership Name

select e1.id,e1.name,e2.id,e2.name from emp e1,emp e2

where e1.leader = e2.id and e1.id=1004;