Oracle数据库：自然连接natural join，using语句，注意避免写交叉连接

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文章目录

Oracle数据库：自然连接natural join，using语句，注意避免写交叉连接
- @[TOC](文章目录)
oracle交叉连接：应该避免写这个东西
oracle自然连接natural join是等值连接的另一种写法
oracle using子句
总结

oracle交叉连接：应该避免写这个东西

直接炸锅了卧槽
一定要避免这么写，炸了………………

select * from employees cross join departments;2889 rows selected

切忌这么写
炸锅

oracle自然连接natural join是等值连接的另一种写法

列名和数据类型
必须都一样

多个字段相等，那就且关系

所以呢，你建立表的时候，就要考虑这个问题
natural join的本质是等值连接的简化
性能一样

懂吧

最好是等值连接
这个一查就是所有列的等

departments表
locations表

SQL> desc locations;
Name           Type         Nullable Default Comments
-------------- ------------ -------- ------- -----------------------------------------------------------------------------------------------------------------------------------------
LOCATION_ID    NUMBER(4)                     Primary key of locations table
STREET_ADDRESS VARCHAR2(40) Y                Street address of an office, warehouse, or production site of a company.
Contains building number and street name
POSTAL_CODE    VARCHAR2(12) Y                Postal code of the location of an office, warehouse, or production site
of a company.
CITY           VARCHAR2(30)                  A not null column that shows city where an office, warehouse, or
production site of a company is located.
STATE_PROVINCE VARCHAR2(25) Y                State or Province where an office, warehouse, or production site of a
company is located.
COUNTRY_ID     CHAR(2)      Y                Country where an office, warehouse, or production site of a company is
located. Foreign key to country_id column of the countries table. SQL> desc departments;
Name            Type         Nullable Default Comments
--------------- ------------ -------- ------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DEPARTMENT_ID   NUMBER(4)                     Primary key column of departments table.
DEPARTMENT_NAME VARCHAR2(30)                  A not null column that shows name of a department. Administration,
Marketing, Purchasing, Human Resources, Shipping, IT, Executive, Public
Relations, Sales, Finance, and Accounting.
MANAGER_ID      NUMBER(6)    Y                Manager_id of a department. Foreign key to employee_id column of employees table. The manager_id column of the employee table references this column.
LOCATION_ID     NUMBER(4)    Y                Location id where a department is located. Foreign key to location_id column of locations table.

试一下

SQL> select d.department_id,d.department_name,l.city from departments d,locations l where d.location_id = l.location_id;DEPARTMENT_ID DEPARTMENT_NAME                CITY
------------- ------------------------------ ------------------------------60 IT                             Southlake50 Shipping                       South San Francisco10 Administration                 Seattle30 Purchasing                     Seattle90 Executive                      Seattle100 Finance                        Seattle110 Accounting                     Seattle120 Treasury                       Seattle130 Corporate Tax                  Seattle140 Control And Credit             Seattle150 Shareholder Services           Seattle160 Benefits                       Seattle170 Manufacturing                  Seattle180 Construction                   Seattle190 Contracting                    Seattle200 Operations                     Seattle210 IT Support                     Seattle220 NOC                            Seattle230 IT Helpdesk                    Seattle240 Government Sales               SeattleDEPARTMENT_ID DEPARTMENT_NAME                CITY
------------- ------------------------------ ------------------------------250 Retail Sales                   Seattle260 Recruiting                     Seattle270 Payroll                        Seattle20 Marketing                      Toronto40 Human Resources                London80 Sales                          Oxford70 Public Relations               Munich27 rows selected

这是等值连接的标准格式
简化一波

SQL> select d.department_id,d.department_name,l.city from departments d natural join locations l;DEPARTMENT_ID DEPARTMENT_NAME                CITY
------------- ------------------------------ ------------------------------60 IT                             Southlake50 Shipping                       South San Francisco10 Administration                 Seattle30 Purchasing                     Seattle90 Executive                      Seattle100 Finance                        Seattle110 Accounting                     Seattle120 Treasury                       Seattle130 Corporate Tax                  Seattle140 Control And Credit             Seattle150 Shareholder Services           Seattle160 Benefits                       Seattle170 Manufacturing                  Seattle180 Construction                   Seattle190 Contracting                    Seattle200 Operations                     Seattle210 IT Support                     Seattle220 NOC                            Seattle230 IT Helpdesk                    Seattle240 Government Sales               SeattleDEPARTMENT_ID DEPARTMENT_NAME                CITY
------------- ------------------------------ ------------------------------250 Retail Sales                   Seattle260 Recruiting                     Seattle270 Payroll                        Seattle20 Marketing                      Toronto40 Human Resources                London80 Sales                          Oxford70 Public Relations               Munich27 rows selected

natural join
的用法就如此
联合俩表相等的列
相当于直接用location_id相同
那通过部门的location_id，去查city信息
所以设计表格的时候，你要设计好
否则就没法用

oracle using子句

using子句
左右表都有id，x列都相同
natural join就会做判断，id和x都得相同

如果用using id
那只看id，不看x
而且你不能用表的别名玩，这个是注意的地方

natural join和using子句是相互排斥的

指定using
不妨设别的可能还相同
那就要限制
行选城市id是1800的

SQL> select d.department_name,l.city from departments d join locations l using(location_id) where location_id=1800;DEPARTMENT_NAME                CITY
------------------------------ ------------------------------
Marketing                      Toronto

natural不能要了哦
另外using加括号（）
然后限定行放屁股

只限定location_id链接，这就是using语句的好处
和natural冲突的

另外你要用location_id，还不表名或者别名了
不能限定了
你看

SQL> select d.department_name,l.city from departments d join locations l using(location_id) where location_id=1800;DEPARTMENT_NAME                CITY
------------------------------ ------------------------------
Marketing                      TorontoSQL> select d.department_name,l.city from departments d join locations l using(location_id) where locations.location_id=1800;
select d.department_name,l.city from departments d join locations l using(location_id) where locations.location_id=1800ORA-00904: "LOCATIONS"."LOCATION_ID": 标识符无效

懂了吧
using，特定列，直接用列名玩

总结

提示：重要经验：

1）
2）学好oracle，即使经济寒冬，整个测开offer绝对不是问题！同时也是你考公网络警察的必经之路。
3）笔试求AC，可以不考虑空间复杂度，但是面试既要考虑时间复杂度最优，也要考虑空间复杂度最优。