3个表

Question

我有三张桌子，顾问，技巧和顾问_技巧。我正在寻找顾问1的所有技能。

我怎样才能在选择和在哪里创建三个关系？

谢谢

SQL - ORACLE DATABASE

CREATE TABLE consultant
(c_id NUMBER(6),
c_last VARCHAR2(20),
c_first VARCHAR2(20),
c_mi CHAR(1),
c_add VARCHAR2(30),
c_city VARCHAR2(20),
c_state CHAR(2),
c_zip VARCHAR2(10),
c_phone VARCHAR2(15),
c_email VARCHAR2(30), 
CONSTRAINT consultant_c_id_pk PRIMARY KEY (c_id));

CREATE TABLE skill
(skill_id NUMBER(3),
skill_description VARCHAR2(50),
CONSTRAINT skill_skill_id_pk PRIMARY KEY (skill_id));

CREATE TABLE consultant_skill
(c_id NUMBER(6),
skill_id NUMBER(3),
certification VARCHAR2(8),
CONSTRAINT consultant_skill_pk PRIMARY KEY (c_id, skill_id),
CONSTRAINT consultant_skill_c_id_fk FOREIGN KEY (c_id) REFERENCES consultant(c_id),
CONSTRAINT consultant_skill_skill_id_fk FOREIGN KEY (skill_id) REFERENCES skill(skill_id));

Answer 1

通常的方法是使用PK来对抗FK，如下所示：

select
  *
from
  consultant
  join
  consultant_skill
    on consultant.c_id = consultant_skill.c_id
  join
  skill
    on consultant_skill.skill_id = skill.skill_id 

根据经验，检查n-1表是否有n个连接以避免笛卡尔连接。