MySQL Niuke brush questions

One 、SQL21 Find the salary increase of active employees since their employment

         1、 subject

        2、 Ideas

         First, find the salary information of the employee when he / she first joined the company and save it in a temporary table , Then find the current salary status of the employee and save it in another table ; Finally, you can get the desired result by subtracting the salaries of the two

        3、 Misinterpretation

                 You can see from the salary table that there are two salaries stored in it , And the result of subtraction is what we need ;

The salary table is adjusted according to emp_no grouping , Then use the salary to reduce the salary （ This is my first thought , In fact, it won't work ） In the previous exercise , What we can do is subtract from column to column （ That is, operations performed by different attributes of the table ） Cannot operate between attributes ; Therefore, it is necessary to separate the previous salary and the current salary into two tables , Then you can subtract ;

        4、 operation

                ① Join two tables , The connection condition is employee number , But there is also a condition that the time should be equal , What you get in this way is the salary at the time of entry , The obtained table is the salary table at the time of employment ;

                ② Join two tables , The connection condition is employee number , But there is also a condition that the time should be ‘9999-01-01’, Because this represents the salary situation of on-the-job employees , That is, the salary table of current employees ;

                ③ In front, I got the salary table of the entry and the current salary table , If you subtract their salary, you get an increase

        5、 Source code

select b.emp_no,(b.salary-a.salary) as growth 
from
(select e.emp_no,s.salary
from employees e left join salaries s 
on e.emp_no = s.emp_no
 and e.hire_date=s.from_date)a
 inner join 
 (
     select e.emp_no,s.salary
     from employees e left join salaries s on e.emp_no = s.emp_no
     where s.to_date = '9999-01-01'
 )b
 on a.emp_no=b.emp_no
 order by growth