我有两个表region(name,number),city(region_number,name,population),我想返回人口最多的前5个城市以及该地区的名称。这是我的查询,但它只返回一个地区的5个城市(本例中为北方),而不是所有地区:
SELECT region.name AS region_name, city.name AS city_name, city.population AS population
FROM region INNER JOIN (SELECT city.name, city.population, city.region_code
FROM county LIMIT 5) AS city
ON region.code = city.region_code;
期望输出:
region_name | city_name | population
------------+----------------------------+------------
NORTH | A | 226338
NORTH | B | 13656
NORTH | C | 9478
NORTH | D | 2464
NORTH | E | 2464
WEST | A | 226338
WEST | B | 13656
WEST | C | 9478
WEST | D | 2464
WEST | E | 2464
提前感谢您的帮助:)
作为一般规则,在没有排序依据的情况下,不要使用limit
。你得到了五个任意的城市。
在MySQL中,您试图做的是有点痛苦。不过,以下措施应该有效:
SELECT r.name AS region_name, c.name AS city_name, c.population AS population
FROM region r INNER JOIN
county c
ON r.code = c.region_code
WHERE 5 >= (SELECT count(*)
FROM county c2
WHERE c2.region_code = c.region_code and c2.population >= c.population
)
ORDER BY region, population DESC;
其中子句计算人口与给定城市相同或更大的城市数量。唯一返回的城市是前5名。
LIMIT是你的SQL关键字朋友,使用SELECT...ORDER BY人口限制5
。