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）
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。
.：
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。
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三 Cartesian Products 笛卡尔积：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
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）。
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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。
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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_nameFROM 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 Zeuss
Listing 3:
更好的写法，使用USING clauseSELECT 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 Grant
SELECT 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 Grant
Listing 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。
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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。
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SELECT * FROM departments WHERE EXISTS (SELECT * FROM employees WHERE departments.department_id = employees.department_id AND > 2500) ORDER BY department_name;
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Present by dylan.