如何在mysql中使用关系时获取行数

Question

我是一个sql的新手，我已经设法选择与许多表有关系的行与下面的sql。我现在有一个第四个表格，其中的项目是模型的“孩子”。模型表（526行）中的“id”和项目表（1505行）中的“model_id”之间存在关系。我现在想要计算items表中与models表中的id具有相同model_id的项，并将它们输出为Items_in_model。

SELECT models.id AS Id, categories.name AS Category, models.category_name 
AS CategoryName, manufacturers.name AS Manufacturer, models.model AS Model, 
models.day_price AS dayprice, models.week_price AS weekprice 
FROM
models, categories, manufacturers
WHERE models.category=categories.id AND 
models.manufacturer=manufacturers.id 
ORDER BY CategoryName ASC

我用这个sql计数工作“独立”

SELECT COUNT(items.id) AS Count FROM items WHERE items.model_id 
LIKE 2

我尝试了很多不同的方法，但这是唯一一个没有给我错误但只输出一行（526）并且计数为（1505，如所有项目）

SELECT models.id AS Id, categories.name AS Category, 
models.category_name AS CategoryName, COUNT(items.id) AS Count, 
manufacturers.name AS Manufacturer, models.model AS Model, 
COUNT(DISTINCT(items.id)) AS ModelCount, models.day_price AS dayprice, 
models.week_price AS weekprice 
FROM
models, categories, manufacturers, items
WHERE models.category=categories.id AND 
models.manufacturer=manufacturers.id AND models.id=items.model_id
ORDER BY CategoryName ASC

提前致谢！

Answer 1

你的目标不是100％明确，但这是我的猜测：

SELECT m.id AS Id, 
    c.name AS Category,
    m.category_name AS CategoryName,
    manufacturers.name AS Manufacturer,
    m.model AS Model, 
    COUNT(i.id) AS ModelCount,
    m.day_price AS dayprice, 
    m.week_price AS weekprice 
FROM models m
LEFT JOIN categories c
ON m.category=c.id
LEFT JOIN manufacturers
ON m.manufacturer=manufacturers.id
LEFT JOIN items i
ON m.id=i.model_id
GROUP BY  m.id

更新不确定你的新问题，但我怀疑你需要额外的JOIN。试试这种方式：

SELECT m.id AS Id, 
    c.name AS Category,
    m.category_name AS CategoryName,
    manufacturers.name AS Manufacturer,
    m.model AS Model, 
    COUNT(i.id) AS ModelCount,
    SUM(IF(i.reservable = 1,1,0)) AS ModelReservableCount,
    m.day_price AS dayprice, 
    m.week_price AS weekprice 
FROM models m
LEFT JOIN categories c
ON m.category=c.id
LEFT JOIN manufacturers
ON m.manufacturer=manufacturers.id
LEFT JOIN items i
ON m.id=i.model_id
GROUP BY  m.id