多表查询

SQL1999国际标准

 

SELECT table1.column,table2.column

From table1

[NATURAL JOIN table2]|

[JOIN table2 USING (column_name)]|

[JOIN table2 ON (table1.column_name = table2.column_name)]|

[LEFT|RIGHT|FULL OUTER JOIN table2 ON (table1.column_name = table2.column_name)]|

[CROSSS JOIN table2]

 

Oracle标准

 

SELECT table1.column,table2.column

FROM table1,table2

WHERE table1.column1 = table.column2

 

例子：

SELECT employee_id,last_name,e.department_id,e.department_id,department_name

FROM employee e,departments d （不能用AS）

WHERE e.department_id = d.department_id

 

如果已经给表定义了别名，使用表的时候必须用表的别名。

 

内连接

表之间的字段信息有关系，等于或者不等于。

 

Natural Join自然连接

 

两张表中必须至少含有一个相同字段并且数据类型相同，才能组合在一起

 

例：

（SQL标准）

SELECT department_id,department_name,location_id,city

FROM departments

NATURAL JOIN locations

（Oracle标准）

SELECT d.department_id,d.department_name,d.location_id,l.city

FROM departments d,locations l

WHERE d.location_id = l.location_id

 

例：

两张表中有相同的字段但数据类型不同，或者指定按表中的一个字段进行连接。

 

（SQL标准)

SELECT employee_id,last_name,location_id,department_id

FROM employees JOIN departments

USING (department_id)

USING字句语法：USING()引用的列，在WHERE里面使用不能加表前缀。

 

（Oracle标准）

SELECT d.department_id,d.department_name,d.location_id,l.city

FROM departments d,locations l

WHERE d.location_id = l.location_id;

 

例：

两张表中没有相同的字段的连接

 

（SQL标准)

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)

(Oracle标准）

SELECT e.employee_id,e.last_name,e.department_id,d.department_id,d.location_id

FROM employees e,departments d

WHERE e.department_id = d.department_id;

 

 

多表连接例(三张表)：

（SQL标准）

SELECT emploee_id,city,department_name

FROM employees e

JOIN departments d

ON d.department_id = e.department_id

JOIN locations l

ON (d.location_id = l.location_id)

(Oracle标准)

SELECT employee_id,city,department_name

FROM employees e,departments d,locations l

WHERE e.department_id = d.department_id AND d.location_id = l.location_id

 

 

条件性JOIN连接

例：

（SQL标准）

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)

AND e.manager id = 149    //或者 WHERE e.manager_id = 149

(Oracle标准)

SELECT d.deparment_id,d.department_name,l.city

FROM departments d,locations l

WHERE d.location_id=l.location_id AND d.department id IN(20,50)

 

自连接

例：

（SQL标准）

SELECT worker.last_name emp,manager.last_name mgr

FROM employees worker JOIN employees manger

ON (worker.manager_id = mgr.employee_id)

（Oracle标准）

SELECT worker.last_name || 'works for' || manager.last_name

FROM employees worker,employeees manager

WHERE worker.manager_id = manager.employee_id

 

Nonequijoins连接（范围比较后，再连接）

例：

 

（SQL标准）

SELECT e.last_name,e.salary,j.grade_level

FROM employees e JOIN job_grades

ON e.salary BETWEEN j.lowest_sal AND j.highest_sal

（Oracle标准）

SELECT e.last_name,e.salary,j.grade_level

FROM employees e JOIN job_grades

WHERE e.salary BETWEEN j.lowest_sal AND j.highest_sal

 

 

外连接

表之间的字段存在没有直接联系，即没有等于或者不等于的关系。

 

左外连接

左表所有记录都显示

 

（Oracle标准）

SELECT table.column,table2.column

FROM table1,table2

WHERE table1.column = table2.column(+)

 

例：

（SQL标准）

SELECT e.last_name,e.department_id,d.department_name

FROM employees e LEFT OUT JOIN departments d

ON (e.department_id = d.department_id)

（Oracle标准）

SELECT e.last_name,e.department_id,d.department_name

FROM employees e,departments d

WHERE e.department_id = d.department_id(+)

 

右外连接

右表所有记录都显示

 

（Oracle标准）

SELECT table.column,table2.column

FROM table1,table2

WHERE table1.column(+)  = table2.column

 

例：

（SQL标准）

SELECT e.last_name,e.department_id,d.department_name

FROM employees e RIGHT OUT JOIN departments d

ON (e.department_id = d.department_id)

（Oracle标准）

SELECT e.last_name,e.department_id,d.department_name

FROM employees e,departments d

WHERE e.department_id(+) = d.department_id

 

 

全外连接

左右表所有记录都显示

 

例：

（SQL标准）

SELECT e.last_name,e.department_id,d.department_name

FROM employees e FULL OUT JOIN departments d

ON (e.department_id = d.department_id)

 

 

笛卡尔乘积(CROSS JOIN)

第一张表的所有记录和第二张表的所有记录组合起来，n*m组合。

产生的原因：

1.内连接或外连接被忽略了

2.内连接或外连接无效

3.随意两张表两两组合，产生大量记录（用于测试）

 

例：

(SQL标准)

SELECT last_name,department_name

FROM employees

CROSS JOIN departments

(Oracle标准)

SELCET last_name,department_name

FROM employees,departments

 

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