先進行sql語句的練習，然後再根據語句進行優化以及建立索引。

# 1檢視僱員編號、名字和部門
select e.emp_no,e.last_name,de.dept_name from 
    employees as e,dept_emp as emp,departments as de where
    e.emp_no = emp.emp_no and de.dept_no=emp.dept_no ;


# 2顯示所有僱員的編號、姓名、工資
select e.emp_no,concat(e.first_name,' ', e.last_name) `name`
 
,sa.salary from
    employees as e,salaries as sa where
    e.emp_no=sa.emp_no limit 100;


# 3查詢在d005號部門工作的僱員
select e.emp_no,concat(e.first_name,' ', e.last_name) `name`,de.dept_name from
    employees as e,dept_emp as emp,departments as de where
    de.dept_no='d005' and de.dept_no=emp.dept_no and e.emp_no=emp.emp_no;
 


# 4要求查詢職位為Engineer和Senior Engineer的僱員姓名(last_name)
select e.last_name from 
    employees as e ,titles as ti where
    e.emp_no=ti.emp_no and
    (ti.title='Engineer' or ti.title='Senior Engineer') ;

select e.last_name from 
    employees as e ,titles as ti where
    e.emp_no=ti.emp_no and
    ti.title in
 
 ('Engineer','Senior Engineer') ;

# 5查詢職位不是Engineer和Senior Engineer的部門編號，僱員部門及姓名。將姓名顯示為(first_name+last_name命名為”Name”)

select concat(e.first_name,' ',last_name) `name`,de.dept_no,de.dept_name from
    employees as e ,dept_emp as emp , departments as de , titles as ti where
    e.emp_no = ti.emp_no and e.emp_no = emp.emp_no and emp.dept_no=de.dept_no and
    ti.title != 'Engineer' and ti.title != 'Senior Engineer' ;

## 會發現資料有重複，那是因為在員工職稱表(title)裡，有些員工是兼顧多個職稱的。

# 6查詢哪些僱員的工資在60000到90000之間
select concat(e.first_name,' ',last_name) `name`,salary from
    employees as e,salaries where
    e.emp_no = salaries.emp_no and
    salaries.salary between 60000 and 90000;

# 7查詢哪些僱員的工資不在60000到90000之間
select concat(e.first_name,' ',last_name) `name`,salary from
    employees as e,salaries where
    e.emp_no = salaries.emp_no and
    salaries.salary not between 60000 and 90000;

# 8查詢first_name以P開頭，後面僅有四個字母的僱員資訊
select distinct concat(e.first_name,' ',last_name) `name` from
    employees as e where
    employees.first_name like "P____";

# 9查詢last_name以K開頭的僱員資訊
select concat(e.first_name,' ',last_name) `name` from
    employees as e where
    e.last_name like "K%";

# 10查詢名字以字母K開頭，以i結尾，並且第三個字母為o的僱員名字(First_name)、職位和所在部門號
select concat(e.first_name,' ',last_name) `name` , emp.dept_no,title from
    employees as e,dept_emp as emp,titles where
    e.emp_no = emp.emp_no and
    e.emp_no = titles.emp_no and
    e.first_name like 'k_o%i';


# 11查詢哪些僱員的職位名不以Se開頭
select concat(e.first_name,' ',last_name) `name` , title from
    employees as e , titles where
    e.emp_no = titles.emp_no and
    titles.title not like 'Se%';

# 12查詢d005號部門裡不是Staff的僱員資訊
select concat(e.first_name,' ',last_name) `name` , e.emp_no,title from
    employees as e ,titles as ti , dept_emp as emp where
    emp.dept_no = 'd005' and
    e.emp_no=emp.emp_no and
    e.emp_no = ti.emp_no and
    ti.title !='Staff';

# 13查詢d005號部門工資大於100000的員工的資訊
select concat(e.first_name,' ',last_name) `name` , e.emp_no,salary from
    employees as e ,salaries , dept_emp as emp where
    e.emp_no = emp.emp_no and
    emp.dept_no='d005'and
    e.emp_no = salaries.emp_no and
    salary >100000;


# 14按字母順序顯示僱員的名字(last_name)
select last_name from 
    employees 
    order by last_name;


# 15按部門編號降序顯示僱員資訊
select concat(e.first_name,' ',last_name) `name` , dept_no from
    employees as e ,dept_emp as emp where
    e.emp_no = emp.emp_no order by
    emp.dept_no desc ;


# 16計算每個部門的平均工資和工資總和.(顯示平均值，總值，部門編號，部門名稱 )
select  avg(salary) , count(salary) ,emp.dept_no,de.dept_name from
    employees as e ,salaries as sa ,dept_emp as emp,departments as de where
    e.emp_no=sa.emp_no and
    emp.emp_no=e.emp_no and
    emp.dept_no =de.dept_no group by
    dept_name ,dept_no order by dept_no;


# 17請算出employees表中每位僱員的平均工資
select first_name,last_name,avg(salary) salary from
    employees as e , salaries as sa where
    e.emp_no=sa.emp_no group by
    first_name,last_name order by salary ;


# 18請算出employees表中所有僱員的平均工資
select avg(salary) from
    employees as e , salaries as sa where
    e.emp_no=sa.emp_no ;


# 19請查詢出employees表中的最低工資的員工資訊

## 典型的慢查詢
select concat(e.first_name,' ',last_name) `name` ,salary  from
    employees as e , salaries as sa where
    e.emp_no=sa.emp_no  order by
    salary  limit 1;

## 正確姿勢
select concat(e.first_name,' ',last_name) `name` ,salary  from
    employees as e , salaries as sa where
    e.emp_no=sa.emp_no  and
    sa.salary= 
    (select min(salary)  from salaries);


# 20請計算出每個部門的平均工資、最高工資和最低工資
select  avg(salary) , count(salary) ,max(salary),min(salary),emp.dept_no,de.dept_name from
    employees as e ,salaries as sa ,dept_emp as emp,departments as de where
    e.emp_no=sa.emp_no and
    emp.emp_no=e.emp_no and
    emp.dept_no =de.dept_no group by
    dept_name ,dept_no order by dept_no; 

# 21查詢薪水發放時間在1986-06-26 ~ 1987-06-25薪水高於46135號僱員並且工種與他相同的僱員情況。
select e.* ,sa.from_date from
    employees as e ,salaries as sa ,titles as ti where
    ti.title in 
    (select title from titles where emp_no=46135) and
    sa.from_date>'1986-06-26' and
    sa.to_date < '1987-06-25' and
    e.emp_no=sa.emp_no and
    e.emp_no = ti.emp_no and
    sa.salary >
     ( select salary from salaries as sa where 
     sa.from_date>'1986-06-26' and sa.to_date< '1987-06-25' and emp_no=46135);

# 22查詢工資在10000到50000之間的僱員所在部門的所有人員的資訊。
select e.*,de.dept_no,de.dept_no,salary from
    employees as e ,dept_emp as emp,departments as de ,salaries as sa where
    sa.salary in (select salary from salaries where salary between 10000 and 50000 ) and
    e.emp_no=emp.emp_no and
    emp.dept_no = de.dept_no and
    sa.emp_no =e.emp_no order by salary;

# 23查詢出在薪水發放時間在1986-06-26 ~ 1987-06-25的員工資訊
###（工號，姓名，性別，薪水，職位)
select e.emp_no , concat(e.first_name,' ',last_name) `name`, e.gender,salary,title from
    employees as e , salaries as sa , titles as ti where
    e.emp_no = sa.emp_no and
    e.emp_no = ti.emp_no and
    sa.from_date > '1986-06-26' and
    sa.to_date < '1987-06-25';