Detailed explanation of the implementation example of group ranking in Mysql tutorial

set @rank =0;
select 
	city ​​,
	score, 
	@rank := @rank+1 rank 
from cs 
order by score desc;

set @rank=0,@price=null;
select cs.* ,
 case when @price = score then @rank 
 when @price := score then @rank := @rank+1 end rank  
 from cs order by score desc;
 -- When the query score value = @price, output @rank,
 -- When they are not equal, assign the score value to @price and output @rank := @rank+1
 
-- or set @rank=0,@price=null;
select 
	a.city,a.score,a.rank 
from 
(select cs.*,
	@rank := if(@p=score,@rank,@rank+1) rank,
	@p := score
from cs 
order by score desc) a;

set @rank=0,@price=null, @z=1;
select 
	a.city,a.score,a.rank 
from 
(select 
	cs.*,
	@rank := if(@p=score,@rank,@z) rank,
	@p := score,@z :=@z+1
from cs 
order by score desc) a;

set @rank=0,@c=null;
select 
	cs.city,cs.score,
	@rank := if(@c = city,@rank+1,1) rank,
	@c := city
from cs 
order by cs.city,cs.score;

set @rank=0,@c=null,@s=null;
select 
	cs.city,cs.score,
	@rank := if(@c=city,if(@s=score,@rank,@rank+1),1) rank ,
	@c := city,
	@s :=score 
from cs 
order by cs.city,cs.score;

set @rank=0,@c=null,@s=null;
select 
	cs.city,cs.score,
	@rank := if(@c=city,if(@s=score,@rank,@rank+1),1) rank ,
	@c := city,
	@s :=score 
from cs 
order by cs.city,cs.score;

set @rank=0,@z=0,@c=null,@s=null;
select a.city,a.score,a.rank from 
(select 
	cs.city city,cs.score score,
	@z := if(@c=city,@z+1,1),
	@rank := if(@c=city,if(@s=score,@rank,@z),1) rank,
	@c := city,
	@s :=score 
from cs 
order by cs.city,cs.score desc) a
where a.rank<=2;

SELECT * FROM cs c
WHERE (
    SELECT count(*) FROM cs
    WHERE c.city=cs.city AND c.score<cs.score )<2
ORDER BY city,score DESC;