的MySQL，@variable和if语句

Question

家伙， 我按照指示，从这个link

我创建的表像这样

CREATE TABLE cities 
(
    city VARCHAR(80), 
    country VARCHAR(80), 
    population INT 
); 

INSERT INTO cities VALUES ('New York', 'United States', 8175133); 
INSERT INTO cities VALUES ('Los Angeles', 'United States', 3792621); 
INSERT INTO cities VALUES ('Chicago', 'United States', 2695598); 

INSERT INTO cities VALUES ('Paris', 'France', 2181000); 
INSERT INTO cities VALUES ('Marseille', 'France', 808000); 
INSERT INTO cities VALUES ('Lyon', 'France', 422000); 

INSERT INTO cities VALUES ('London', 'United Kingdom', 7825300); 
INSERT INTO cities VALUES ('Birmingham', 'United Kingdom', 1016800); 
INSERT INTO cities VALUES ('Leeds', 'United Kingdom', 770800);  

当我运行此查询

SELECT city, country, population 
    FROM 
    (SELECT city, country, population, 
       @country_rank := IF(@current_country = country, @country_rank + 1, 1) AS country_rank, 
       @current_country := country 
    FROM cities 
    ORDER BY country, population DESC 
    ) ranked 
    WHERE country_rank <= 2; 

它doesn”

t给我每个国家2个最大的城市

是否有我错过了什么？感谢

Answer 1

添加下面的两个线路码的代码之前，它会工作。

select @current_country:=NULL; 
select @country_rank:=0; 


SELECT city, country, population 
    FROM 
    (SELECT city, country, population, 
      @country_rank := IF(@current_country = country, @country_rank + 1, 1) AS country_rank, 
      @current_country := country 
    FROM cities 
    ORDER BY country, population DESC 
    ) ranked 
    WHERE country_rank <= 2; 

Answer 2

尝试增加其他子查询初始化变量：

SELECT city, country, population 
FROM 
    (SELECT city, country, population, 
      @country_rank := IF(@current_country = country, @country_rank + 1, 1) AS country_rank, 
      @current_country := country 
    FROM cities 
    ORDER BY country, population DESC 
) ranked 
CROSS JOIN (SELECT @country_rank := 0, $current_country := '') vars 
WHERE country_rank <= 2;