SQL - 组合查询

Question

我有两个完全按照我想要的查询，但是，我想尝试将它们组合起来。

查询1：

SELECT DISTINCT categories.id FROM categories 
    INNER JOIN product_categories 
        ON product_categories.category_id=categories.id 
    INNER JOIN products 
        ON product_categories.product_id=products.id 
    WHERE 
        categories.is_sub_cat='1' AND 
        categories.is_paused!='1' AND 
        products.nexus='1'

查询2：

SELECT DISTINCT categories.top_cat FROM categories 
    INNER JOIN product_categories 
        ON product_categories.category_id=categories.id 
    INNER JOIN products 
        ON product_categories.product_id=products.id 
    WHERE 
        categories.top_cat!='0' AND 
        categories.is_paused!='1' AND 
        products.nexus='1'

是否可以将这些组合成1个查询，或者我应该在完成两个查询之后将结果数组合并到PHP中？

Answer 1

您可以使用UNION合并两个查询

SELECT DISTINCT categories.id FROM categories 
    INNER JOIN product_categories 
        ON product_categories.category_id=categories.id 
    INNER JOIN products 
        ON product_categories.product_id=products.id 
    WHERE 
        categories.is_sub_cat='1' AND 
        categories.is_paused!='1' AND 
        products.nexus='1'
UNION 

SELECT DISTINCT categories.top_cat FROM categories 
    INNER JOIN product_categories 
        ON product_categories.category_id=categories.id 
    INNER JOIN products 
        ON product_categories.product_id=products.id 
    WHERE 
        categories.top_cat!='0' AND 
        categories.is_paused!='1' AND 
        products.nexus='1'

Answer 2

你可以用的时候用

SELECT case when (categories.is_sub_cat='1' AND  categories.is_paused!='1' AND 
        products.nexus='1') then categories.id
        else categories.top_cat end  as id
    FROM categories 
    INNER JOIN product_categories 
        ON product_categories.category_id=categories.id 
    INNER JOIN products 
        ON product_categories.product_id=products.id 

Answer 3

您可以将查询组合为

SELECT DISTINCT categories.id, categories.top_cat FROM categories 
    INNER JOIN product_categories 
        ON product_categories.category_id=categories.id 
    INNER JOIN products 
        ON product_categories.product_id=products.id 
    WHERE 
        ( categories.is_sub_cat='1' OR categories.top_cat!='0' ) AND 
        categories.is_paused!='1' AND 
        products.nexus='1'

或者，如果列categories.id和categories.top_cat具有相同的数据类型，或者能够转换为相同的数据类型，则可以使用UNION ALL 。

Answer 4

是的，UNION似乎是一个合法的选择。 对于你的情况，它应该没关系，因为你是从同一个表中选择，但作为旁注，你应该知道，当使用UNION时，你需要在表中具有相同数量的列（即如果它们不同）。

Answer 5

您可以使用WHERE子句中的条件执行此操作：

SELECT DISTINCT c.id
FROM categories c INNER JOIN
     product_categories pc
     ON pc.category_id = c.id INNER JOIN
     products p
     ON pc.product_id = p.id 
WHERE p.nexus = 1 AND
      c.is_paused <> 1 AND
      (c.is_sub_cat = 1 OR
       c.top_cat <> 0 
      );

注意：

• 表别名使查询更易于编写和读取。
• 假设列是数字，请不要将值与字符串进行比较。 尽量避免类型转换问题。