查询微调

Question

我对sql相当陌生，并且想要询问您对此查询的帮助。有没有另一种方法来重写这个？

Select * 
From emp 
Where emp_no IN (
    Select emp_no 
    From dept_emp 
    Where dept_no = 'd002' 
    ); 

任何帮助，非常感谢。

谢谢。

Answer 1

您可以使用exists：

select * 
from emp 
where exists(
    select 1 from dept_emp where dept_emp.emp_no = emp.emp_no and dept_no = 'd002' 
) 

也说不定inner join工作：

select emp.* 
from emp 
join dept_emp 
on dept_emp.emp_no = emp.emp_no 
and dept_no = 'd002' 

Answer 2

您可以使用内部联接

select * 
from emp 
inner join (
    Select emp_no 
    From dept_emp 
    Where dept_no = 'd002' 

) t on emp_no.id = t.emp_no 

或无子查询

select * 
from emp 
inner join dept_emp on emp_no.id = dept_emp.emp_no 
    and dept_emp.dept_no = 'd002' 

Answer 3

您可以使用加入这样的：

select * from emp e, dept_emp d where e.emp_no = d.emp_no and d.dept_no = 'd002'

Answer 4

 
- If have the relation between two tables best-way you can used join. other wise you can used inner query (sub select) 
- When you used join in RDBMS it will faster that inner query. 
- join must be indexed based like: primary key and foreign key. Other wise executing cost will be high. non-index query time consumed maximum.