SQL 基礎(chǔ)之子查詢（十一）

子查詢:類型、語法、和注意事項

使用子查詢能解決哪些問題？

子查詢語法：

select select_list from table where expr operator (select select_list from table);

• 子查詢（內(nèi)查詢）在主查詢（外查詢）之前執(zhí)行。

• 主查詢使用子查詢結(jié)果。

• 位置：select，where，from，having

1、查詢誰的工資比Abel高

select last_name, salary from employees 

where salary >

(select salary

from employees

where last_name = 'Abel');

使用子查詢注意事項

• 子查詢要包含在括號內(nèi)。

• 將子查詢放在比較條件的右側(cè)增強(qiáng)可讀性（子查詢可以出現(xiàn)在比較運(yùn)算符的兩側(cè)）

• 單行操作符對應(yīng)單行子查詢，多行操作符對應(yīng)多行子查詢

單行子查詢:

– 子查詢中的組函數(shù)

– HAVING 子句中的子查詢

• 只返回一行

• 使用單行比較操作符

  
  

select last_name, job_id, salary from employees

where job_id  in  (select job_id from employees

where last_name like  'Taylor')

and salary in

(select salary

from employees

where last_name like 'Taylor');

在子查詢中使用組函數(shù)

select last_name, job_id, salary from employees where 

salary = (select min(salary) from employees);

子查詢中的HAVING 子句

• 首先執(zhí)行子查詢

• 向主查詢中的 HAVING 子句返回結(jié)果

select department_id, min(salary)

from employees

group by department_id

having min(salary) >

(select min(salary)

from employees

where department_id = 50);

多行子查詢使用單行比較符，以下為錯誤寫法

select employee_id, last_name

from employees

where salary =

(select min(salary)

from employees

group by department_id);

子查詢中的空值問題

select last_name, job_id from employees

where job_id =

(select job_id from employees

where last_name = 'haas');

多行子查詢

– 使用 ALL 或 ANY

• 返回多行。

• 使用多行比較操作符。

使用范例：

在多行子查詢中使用 ANY 

select employee_id, last_name, job_id, salary

from employees  where salary < any

(select salary

from employees

where job_id = 'IT_PROG')

and job_id < > 'IT_PROG';

在多行子查詢中使用 ALL  操作符

select employee_id, last_name, job_id, salary

from employees

where salary < all

(select salary

from employees

where job_id = 'IT_PROG')

and job_id <> 'IT_PROG';

子查詢中的空值

select emp.last_name

from employees emp

where emp.employee_id not in

(select mgr.manager_id

from employees mgr);

1、HR 部門的同事想要你幫忙寫一個 SQL 語句，該 SQL 語句可以傳入一個值（員工的 last_name），然后返回結(jié)果是該員工同一部門同事的 last_name 和 hire_date，且要求該員工不在返回結(jié)果中。

舉個例子，如果用戶輸入”Zlotkey”，結(jié)果就會返回除了 Zlotkey 之外的同一部門的其他同事的

last_name 和 hire_date.

select last_name,hire_date

from employees

where department_id =(select department_id from employees

where last_name= '&&enter_name')

and last_name < > '&enter_name';

2、請查詢出所有高于平均工資的員工的 employee_id,last_name,salary，并將最終結(jié)果根據(jù)salary 降序排列。

select employee_id,last_name,salary 

from employees 

where salary > (select avg(salary) 

            from employees) 

order by salary;

3、請寫一條 SQL 語句，要求查詢出那些同一部門員工 last_name 里面包含字母”u”的員工的employee_id，last_name。

select employee_id,last_name from employees where department_id in (select department_id from employees where last_name like '%u%');

4、請幫助HR部門的同事查出所有部門location_id是1700的員工的last_name,department_id,job_id。

select last_name,department_id,job_id

from employees

where department_id in(select department_id

from departments

where location_id=1700);

讓用戶可以選擇輸入一個 location_id,然后輸出結(jié)果。

select last_name,department_id,job_id

from employees

where department_id in(select department_id

from departments

where location_id=&enter_location);

5、請查出所有需要向 King 匯報的員工的 last_name 以及 salary

select last_name,salary,manager_id

from employees

where manager_id = (select employee_id

from employees

where last_name like 'King' and manager_id is null);

6、請查出所有是執(zhí)行部（Executive）的員工的 department_id,last_name,job_id

select department_id,last_name,job_id

from employees

where department_id in(select department_id

from departments

where department_name like 'Executive');

7、請查出比 department_id 是 60 的任何員工薪水高的所有的員工的 last_name。

select department_id,last_name,salary from employees 

where salary > any 

(select salary from employees

where department_id=60);

8、查詢所有高于平均工資，并且同一部門員工 last_name 里面包含字母”u”的員工的 employee_id,last_name,salary。

select employee_id,last_name,salary

from employees

where department_id in(select department_id

from employees

where last_name like '%u%')

and salary > (select avg(salary) from employees);