Variable value is not incrementing

All we need is an easy explanation of the problem, so here it is.

Why is the country_rank value 1 for each row, which I assume is due to STRCMP returing false everytime. I think @current_country is not set correctly but dont know why.

Table:

sn country city
1  AAA     aaa
2  BBB     bbb
3  AAA     ccc

Output Expected:

country city country_rank
AAA     ccc  1
AAA     aaa  2
BBB     bbb  1

Query:

SELECT country,city,
@country_rank := IF(STRCMP(country,@current_country) = 0,  @country_rank + 1, 1) AS country_rank,
@current_country := country
FROM table  
ORDER BY country,sn  DESC

The query is supposed to rank the cities in each country as per sn.
MYSQL 5.6

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

SELECT country,city,
       @country_rank :=
          IF(STRCMP(country,@current_country) = 0,  @country_rank + 1, 1)
          AS country_rank,
       @current_country := country  setctry
  FROM  cities,
        (select @current_country="", @country_rank=0) init
  ORDER BY country,sn  DESC

cfr. SQL Fiddle http://sqlfiddle.com/#!9/c388ef7/2

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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