Use of MySQL variables

  Window functions can be viewed by clicking this link

•   Error prone points of variable use
  • about SET, have access to = or := As allocator
  • Use SELECT Defining user variables can only use := As allocator ( Because in Africa SET In the sentence = Be treated as one ‘ Comparison operator ’.
• Through variables and if Statement to implement the window function (1):dense_rank()

SELECT
    salary,
    rk AS `RANK` 
FROM
    (
    SELECT
        salary,
    IF
        ---- If @pres and salary equal , be @currank otherwise @currank be equal to @currank + 1
        ( @pres = salary, @currank, @currank := @currank + 1 ) AS rk,
        @pres := salary 
    FROM
        employee,(
        SELECT
            ---- Declare variables @pres by -1,@currank by 0 
            @pres := - 1,
            @currank := 0 
        ) c1 
    ORDER BY salary DESC ) s

•   Through variables and case when Statement to implement the window function (2):dense_rank()

    SELECT
        salary,
        ( CASE WHEN @pres = salary THEN @currank WHEN @pres := salary THEN @currank := @currank + 1 END ) AS `RANK` 
    FROM
        employee,
        ( SELECT @pres := - 1, @currank := 0 ) AS c1 
    ORDER BY
    salary DESC;
   

•   adopt Count And DISTINCT Implement window functions (3):dense_rank()

SELECT
    s.salary,
    ( SELECT COUNT( DISTINCT salary ) FROM employee e WHERE e.salary >= s.salary ) AS c1 
FROM
    employee s 
ORDER BY
    salary DESC; 

•    adopt group by,order by And variable implementation window function (4):dense_rank()

select 
    s.salary,
    r.rank
from 
    employee s,
    (
        select 
            tmp1.salary,
            (@rownum:[email protected]+1) as `Rank`
        from 
            (select salary from employee group by salary order by salary desc) as tmp1,
            (select @rownum:=0) tmp2
    ) as r
where 
    s.salary=r.salary 
    order by s.salary desc;

• adopt if Function to solve the following deformation problem

 SELECT
    DISTINCT s.Num 
FROM
    (
    SELECT
        t.*,
    IF
        ( @before_num = t.Num, @times := @times + 1, 1 ) AS times,  
        @before_num := t.Num 
    FROM
        ( SELECT @before_num := NULL, @times := 1 ) r,
    test t 
    ) s
    
    WHERE s.times >=3