多表連接

多表連接

select table1.aaa,table2.bbb from table1

–cross join table2

–natural join table2

–join table2 using (xxx)

–join table on table1.table1_id = table2.table2 _id

–left/right/full (outer) join table2 on table1.table1_id = table2.table2 _id

• 內連接（等值連接）

  
  select e.last_name,d.department_name from employees e,departments d where e.department_id=d.department_id
  =
  
  select e.last_name,d.department_name from employees e join departments d on e.department_id=d.department_id

• 外連接 left/right join 表名 on

  left join 表名 on
  
  
  select e.last_name,d.department_name from employees e left join departments d on e.department_id=d.department_id
  =
  
  使用(+), 帶(+)的爲補充表
  select e.last_name,d.department_name from employees e,departments d where e.department_id=d.department_id(+)

• 自連接

  select worker.last_name ||' works for '|| manager.last_name from employees worker,employees manager where worker.manager_id = manager.employee_id

• 自然連接 natural 表名 join

  以兩個表中名字相同的列爲條件創建等值連接

  如果有相同名字的列數據類型不同，返回一個錯誤

  
  select * from employees natural join departments   
  =
  
  select * from employees e,departments d where e.department_id=d.department_id and d.manager_id=e.manager_id
  
  using指定等值連接用到的列   join 表名 using ，  natural join 與 using 是互相排斥的
  在引用列不要使用表名或別名
  
  select * from employees join departments using (department_id) 
  =
  
  select * from employees e,departments d where e.department_id=d.department_id
  
  select * from employees join departments using (department_id) where department_id = 10
  
  select * from employees join departments d using (department_id) where d.department_id = 10 --錯誤的

• 交叉連接 cross 表名 join

  兩個表的交叉乘積，該連接和兩個表之間的笛卡爾乘積是一樣的

  
  select last_name,department_name from employees cross join departments
  =
  
  select last_name,department_name from employees,departments

• 全外連接 full join 表名 on

  返回兩個表中的行，即使不匹配的行也會返回

  select * from employees e full join departments d on e.department_id=d.department_id

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