oracle连接查询详解
连接查询是数据库查询语句中使用频率很高的查询方式，下面根据Oracle提供的官方文档学习一下连接查询：
一 Equijoins 等值连接：
等值连接是条件连接在连接运算符为“=”号时的特例。（相对于非等值连接，如：between...and条件）
三 Cartesian Products 笛卡尔积：
它是从关系R与S的广义笛卡尔积中选取A，B属性值相等的那些元组。
e.g.：
SQL> select e.employee_id, e.last_name,
2 e.department_id, d.department_id,
3 d.location_id
4 from employees e, departments d
5 where e.department_id = d.department_id;
EMPLOYEE_ID LAST_NAME DEPARTMENT_ID DEPARTMENT_ID LOCATION_ID
----------- ------------------------- ------------- ------------- -----------
198 OConnell 50 50 1500
199 Grant 50 50 1500
200 Whalen 10 10 1700
201 Hartstein 20 20 1800
......二 Self joins 自连接：
自连接(self join)是SQL语句中经常要用的连接方式，使用自连接可以将自身表的一个镜像当作另一个表来对待，从而能够得到一些特殊的数据。
e.g.
SELECT e1.last_name||' works for '||e2.last_name
"Employees and Their Managers"
FROM employees e1, employees e2
WHERE e1.manager_id = e2.employee_id
AND e1.last_name LIKE 'R%'
ORDER BY e1.last_name;
Employees and Their Managers
-------------------------------
Rajs works for Mourgos
Raphaely works for King
Rogers works for Kaufling
Russell works for King
三 Cartesian Products 笛卡尔积：
If two tables in a join query have no join condition, then Oracle Database returns their Cartesian product.
如果2个表连接查询而没有连接条件，则oracle返回他们的笛卡尔积。即oracle返回一个表里每一行与另一个表每一行的组合（15 X 4）。
e.g.
SCOTT@orcl> select count(1) from emp;
COUNT(1)
----------
15
SCOTT@orcl> select count(1) from dept;
COUNT(1)
----------
4
SCOTT@orcl> select count(1) from emp, dept;
COUNT(1)
----------
60四 Inner Joins 内连接：
An inner join (sometimes called a simple join) is a join of two or more tables that returns only those rows that satisfy the join condition.
内连接也叫简单连接，是2个或更多表的关联并且仅返回那些满足连接条件的行。
e.g.
select e.employee_id, e.last_name,
e.department_id, d.department_id,
d.location_id
FROM employees e JOIN departments d
ON e.department_id = d.department_id;
五 Outer joins 外连接：
简单连接的扩展。分为左外连接（Left outer joins）、右外连接（Right outer joins）、全外连接（Full outer joins）
和Partitioned Outer Joins（分区外连接，用于数据仓库）
Listing 1:
SELECT d.department_id, e.last_name
FROM departments d LEFT OUTER JOIN employees e
ON d.department_id = e.department_id
ORDER BY d.department_id, e.last_name;另外一种写法：
SELECT d.department_id, e.last_name
FROM departments d, employees e
WHERE d.department_id = e.department_id(+)
ORDER BY d.department_id, e.last_name;Oracle 官方推荐使用第一种写法。
Listing 2:
SELECT d.department_id, e.last_name
FROM departments d RIGHT OUTER JOIN employees e
ON d.department_id = e.department_id
ORDER BY d.department_id, e.last_name;
DEPARTMENT_ID LAST_NAME
------------- -------------------------
. . .
110 Higgins
110 Gietz
Grant
ZeussListing 3:
SELECT d.department_id as d_dept_id, e.department_id as e_dept_id,
e.last_name
FROM departments d FULL OUTER JOIN employees e
ON d.department_id = e.department_id
ORDER BY d.department_id, e.last_name;
D_DEPT_ID E_DEPT_ID LAST_NAME
---------- ---------- -------------------------
. . .
110 110 Gietz
110 110 Higgins
. . .
260
270
999 Zeuss
GrantSELECT department_id AS d_e_dept_id, e.last_name
FROM departments d FULL OUTER JOIN employees e
USING (department_id)
ORDER BY department_id, e.last_name;
D_E_DEPT_ID LAST_NAME
----------- -------------------------
. . .
110 Higgins
110 Gietz
. . .
260
270
999 Zeuss
GrantListing 4:
CREATE TABLE inventory (time_id DATE,
product VARCHAR2(10),
quantity NUMBER);
INSERT INTO inventory VALUES (TO_DATE('01/04/01', 'DD/MM/YY'), 'bottle', 10);
INSERT INTO inventory VALUES (TO_DATE('06/04/01', 'DD/MM/YY'), 'bottle', 10);
INSERT INTO inventory VALUES (TO_DATE('01/04/01', 'DD/MM/YY'), 'can', 10);
INSERT INTO inventory VALUES (TO_DATE('04/04/01', 'DD/MM/YY'), 'can', 10);
SELECT times.time_id, product, quantity FROM inventory
PARTITION BY (product)
RIGHT OUTER JOIN times ON (times.time_id = inventory.time_id)
WHERE times.time_id BETWEEN TO_DATE('01/04/01', 'DD/MM/YY')
AND TO_DATE('06/04/01', 'DD/MM/YY')
ORDER BY 2,1;
TIME_ID PRODUCT QUANTITY
--------- ---------- ----------
01-APR-01 bottle 10
02-APR-01 bottle
03-APR-01 bottle
04-APR-01 bottle
05-APR-01 bottle
06-APR-01 bottle 10
06-APR-01 bottle 8
01-APR-01 can 10
01-APR-01 can 15
02-APR-01 can
03-APR-01 can
04-APR-01 can 10
04-APR-01 can 11
05-APR-01 can
06-APR-01 can
15 rows selected.更好的写法：
SELECT time_id, product, LAST_VALUE(quantity IGNORE NULLS)
OVER (PARTITION BY product ORDER BY time_id) quantity
FROM ( SELECT times.time_id, product, quantity
FROM inventory PARTITION BY (product)
RIGHT OUTER JOIN times ON (times.time_id = inventory.time_id)
WHERE times.time_id BETWEEN TO_DATE('01/04/01', 'DD/MM/YY')
AND TO_DATE('06/04/01', 'DD/MM/YY'))
ORDER BY 2,1;
TIME_ID PRODUCT QUANTITY
--------- ---------- ----------
01-APR-01 bottle 10
02-APR-01 bottle 10
03-APR-01 bottle 10
04-APR-01 bottle 10
05-APR-01 bottle 10
06-APR-01 bottle 8
06-APR-01 bottle 8
01-APR-01 can 15
01-APR-01 can 15
02-APR-01 can 15
03-APR-01 can 15
04-APR-01 can 11
04-APR-01 can 11
05-APR-01 can 11
06-APR-01 can 11
15 rows selected.六 Anti-joins 反连接：
返回不满足子查询条件（NOT IN）的左表记录。
e.g.
SELECT * FROM employees
WHERE department_id NOT IN
(SELECT department_id FROM departments
WHERE location_id = 1700)
ORDER BY last_name;七 Semi-joins 半连接：
返回匹配exists子查询的左侧没有重复的行。
e.g.
SELECT * FROM departments
WHERE EXISTS
(SELECT * FROM employees
WHERE departments.department_id = employees.department_id
AND employees.salary > 2500)
ORDER BY department_name;
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Present by dylan.