#多表查詢
#出現了笛卡爾積的錯誤
select employee_id,last_name,department_name
from employees,departments; #2889
select 2889 / 107
from dual; # 27
select department_id
from departments; #27
#正確的：
select employee_id,last_name,department_name
from employees,departments
#多表的連線條件
where employees.`department_id` = departments.`department_id`;
#如果查詢的字段在多個表**現，則必須顯式的指明此欄位所屬的表
#sql優化
select employees.employee_id,employees.last_name,departments.department_name,employees.department_id
from employees,departments
where employees.`department_id` = departments.`department_id`;
#表的別名,可以在select、where中使用。
select e.employee_id,e.last_name,d.department_name,e.department_id
from employees e,departments d
where e.`department_id` = d.`department_id`;
#三個表的連線練習：
select employee_id,last_name,department_name,city
from employees e,departments d,locations l
where e.`department_id` = d.`department_id`
and d.`location_id` = l.`location_id`;
#結論：n個表做查詢操作，則至少有 n - 1個連線條件。
#################################
/*多表查詢的總結：分類
等值連線 vs 非等值連線
自連線 vs 非自連線
內連線 vs 外連線
*/#非等值連線
select employee_id,last_name,salary,grade_level
from employees e,job_grades j
where e.`salary` between j.`lowest_sal` and j.`highest_sal`;
#自連線
select emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
from employees emp,employees mgr
where emp.`manager_id` = mgr.`employee_id`;
#內連線:合併具有同一列的兩個以上的表的行, 結果集中不包含乙個表與另乙個表不匹配的行
select e.employee_id,e.last_name,d.department_name,e.department_id
from employees e,departments d
where e.`department_id` = d.`department_id`; #106行
#左外連線：除了查詢到兩個表滿足連線條件的行以外，還返回左表中不滿足條件的行
#右外連線：除了查詢到兩個表滿足連線條件的行以外，還返回右表中不滿足條件的行
#字眼：查詢所有的。。。。
#sql 99語法中提到了如何使用左外連線、右外連線
#sql 99語法實現內連線：
select e.employee_id,e.last_name,d.department_name,e.department_id
from employees e join departments d
on e.`department_id` = d.`department_id`;
select e.employee_id,e.last_name,d.department_name,e.department_id,city
from employees e join departments d
on e.`department_id` = d.`department_id`
join locations l
on d.`location_id` = l.`location_id`;
#sql 99語法實現左外連線：
select e.employee_id,e.last_name,d.department_name,e.department_id
from employees e left outer join departments d
on e.`department_id` = d.`department_id`;
#sql 99語法實現右外連線：
select e.employee_id,e.last_name,d.department_name,d.department_id
from employees e right join departments d
on e.`department_id` = d.`department_id`;
# 滿外連線:mysql不支援full join
select e.employee_id,e.last_name,d.department_name,d.department_id
from employees e full join departments d
on e.`department_id` = d.`department_id`;
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