According connection table to divide, including:
en:
equijoins
non-equivalent connections
from the connection
outer join:
Left outer join (left join)
right outer join (right connection) |
fully connected (this does not write, very few use!)
Case: Find out the name of every employee of the department, required to display employee name and department name.
select e.ename,d.dname from emp e,dept d where e.deptno=d.deptno;
Cartesian product phenomenon: when two table join query, without any conditions limiting the final number of query result is the product of two tables of the number of records.
About alias tables:
the SELECT e.ename, d.dname from emp E, the dept d;
alias tables What are the benefits?
First: implementation of high efficiency.
Second: readable.
The equivalent of the connector connection: most important feature: the condition is equivalent relationship.
SQL92: (too old, without a)
SELECT
E ename, DNAME D..
From
EMP E, Dept D
WHERE
E = D DEPTNO DEPTNO;..
SQL99: // innex can be omitted, with the inner purpose is a good readability - some.
(Common)
SELECT
E ename, DNAME D..
From
EMP E
Inner the Join
Dept D
ON
e.deptno DEPTNO = D;.
Syntax:
A
the Join
B
ON
connection conditions
where
...
SQL99 syntax structure more clearly some of them: the connection condition table and the subsequent condition where separated
【example】:
select e.ename,d.dname from emp e join dept d on e.deptno=d.deptno;
Nonequijoins connection of the connector: characterized by the largest: the relationship is non-connected conditions equivalent relationship. .
Case: identify each employee's salary level, employees required to display the name, salary, wage scale.
select s.grade,e.ename,e.sal from emp e join salgrade s on e.sal between s.losal and s.hisal;
Since the connection: The greatest feature is: a table seen as two tables. Connect your own.
Case: identify each employee's superiors, employees required to display the corresponding name and the name of leadership.
The following table:
select e.ename,e.mgr,p.ename,p.empno from emp e join emp p on e.mgr=p.empno;
Outer join?
What is the outer join, and the connector What is the difference?
En:
Suppose tables A and B are connected, the connection used, all tables A and B can be matched to the table on the recording _ check out, that is the connection.
AB two tables is not the main points of the deputy, two tables are equal.
Outer connecting:
Let A and B tables are connected, using external connections, then, AB two tables with a table is a primary table, a table is a sub-table, the main query data in the main table, incidentally query sub-table, when the secondary It does not match the data in the table and the data in the main table. on the side tables automatically matching simulated NULL.
? Classification outer join
left outer join (left join): Indicates the left side of this table is the main table.
Right outer connector (Right link): this table indicates the right is the main table.
External connection:
SELECT
a.ename 'employee', b.ename 'leaders'
from
EMP A
lelft the Join
EMP B
ON
a.mgr EMPNO = B;.
select e.ename,e.mgr,p.ename,p.empno from emp e left join emp p on e.mgr=p.empno;
How three tables join query?
Grammar:
A
the Join
B
on
....
join
C
on
.....
represents: A table and the first table join Table B, A, and c in Table continued connection after the connection table. .
Case: identify each employee's department name and salary grades
select
e.ename,d.dname,s.grade
from
emp e
join
dept d
on
e.deptno=d.deptno
join
salgrade s
on e.sal between s.losal and s.hisal;
Case: Find out the name of each employee's department, wage levels, and higher collar
select
d.dname,s.grade,e1.mgr
from
emp e
join
dept d
on
e.deptno=d.deptno
join
salgrade s
on
e.sal between s.losal and s.hisal
left join
emp e1
on
e.mgr = e1.empno;
