Question

我有3个表：products，products_description，products_to_categories

这是我的sql：

SELECT `products`.`products_model`, 
       `products_description`.`products_name` 
FROM   products, 
       products_description, 
       products_to_categories 
WHERE  `products`.`products_id` = `products_description`.`products_id` = 
                                         `products_to_categories`. `products_id` 
       AND `products_to_categories`.`categories_id` <> '91' 
ORDER  BY `products`.`products_model` DESC

所有三个表都有products_id作为主键，我试图基于此链接所有三个。我想我需要加入，因为我似乎无法让所有三个在一个声明中相等？

Answer 1

你可以通过JOIN来做到这一点，但是如果你想使用Where语句那么好，但是你必须使用 AND 来实现它

SELECT `products`.`products_model`, `products_description`.`products_name`
FROM products, products_description, products_to_categories
WHERE `products`.`products_id` =  `products_description`.`products_id` 
   AND `products`.`products_id`= `products_to_categories`.`products_id` 
   AND `products_to_categories`.`categories_id` <> '91' 
ORDER BY `products`.`products_model` DESC

Answer 2

select p.`products_model`,
    pd.`products_name`
from products p
inner join products_description pd on p.`products_id` = pd.`products_id`
inner join products_to_categories pc on p.`products_id` = pc.`products_id`
where pc.`categories_id` <> '91'
order by p.`products_model` desc

Answer 3

尝试

SELECT products_model, products_name
  FROM products
       NATURAL JOIN products_description
       NATURAL JOIN products_to_categories;

Answer 4

无论如何，更现代的JOIN通常比旧的隐式连接语法更受欢迎。它提供了关于表格如何相关的更具可读性的图片，并通过仅包括WHERE中的实际限制条件来允许更清晰的WHERE子句。

SELECT 
  `products`.`products_model`,
  `products_description`.`products_name`
FROM 
  products
  JOIN products_description ON products.product_id = products_description.product_id
  JOIN products_to_categories ON products.product_id = products_to_categories.product_id
WHERE  `products_to_categories`.`categories_id` <> '91' 
ORDER BY `products`.`products_model` DESC

Answer 5

尽量不要在FROM子句中使用逗号。它是sql标准的更旧版本的延续，并不再被认为是一个好习惯。几乎就在您使用TableA,TableB语法的任何地方，您应该学会使用TableA INNER JOIN TableB ON代替。

在这种情况下，你可以像这样逃避：

SELECT `products`.`products_model`, `products_description`.`products_name`
FROM products, products_description, products_to_categories
WHERE `products`.`products_id` =  `products_description`.`products_id` 
    AND `products`.`products_id` = `products_to_categories`. `products_id` 
    AND `products_to_categories`.`categories_id` <> '91' 
ORDER BY `products`.`products_model` DESC

但是，你真的应该学会这样写：

SELECT `products`.`products_model`, `products_description`.`products_name`
FROM products
INNER JOIN products_description ON products.products_id = `products_description`.`products_id`
INNER JOIN `products_to_categories` ON `products_to_categories`.`products_id` = `products`.`products_id`
WHERE `products_to_categories`.`categories_id` <> '91' 
ORDER BY `products`.`products_model` DESC

Answer 6

    SELECT `products`.`products_model`, `products_description`.`products_name`

FROM products，products_description，products_to_categories 在products。products_id = products_description。products_id和products_to_categories。 products_id = products。products_id和products_description。products_id = products_to_categories。 products_id products_to_categories。 products_id和products_to_categories。categories_id＆lt;＆gt; '91' ORDER BY products。products_model DESC

试试这个我觉得它比你的查询更好用

我需要加入吗？

6 个答案: