Question

我正在使用这个MySQL语句来查找社区的平均财产评估。问题是我必须为每个邻域添加一个SELECT语句 - 并且有很多邻域。有没有办法做到这一点，而无需指定'RIVER FRONT'或'OLD TOWN'？有更有效的方法吗？

SELECT AVG(property_table.assessment) 
FROM property_table, neighborhood_table  
WHERE property_table.id = neighborhood_table.id AND neighborhood_table.neighborhood = 'RIVER FRONT'
    UNION ALL
SELECT AVG(property_table.assessment) 
FROM property_table, neighborhood_table  
WHERE property_table.id = neighborhood_table.id AND neighborhood_table.neighborhood = 'OLD TOWN'

Answer 1

这是一个标准的SQL聚合语句，显示所有邻域的平均值左连接允许您获得零值，其中邻域中不存在属性。

SELECT 
    n.neighborhood, COALESCE(AVG(p.assessment), 0)
FROM
    neighborhood_table n
    LEFT JOIN 
    property_table p ON p.id = n.id
GROUP BY 
    n.neighborhood;

Answer 2

SELECT AVG(property_table.assessment), neighborhood_table.neighborhood
FROM property_table
INNER JOIN neighborhood_table ON property_table.id = neighborhood_table.id
WHERE neighborhood_table.neighborhood IN ('RIVER_FRONT', 'OLD TOWN')
GROUP BY neighborhood_table.neighborhood;

MySQL平均值：使语句更有效

2 个答案: