SQL - Combining Queries

Question

I have two queries that do exactly as I want, however, I want to try combine these.

Query 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'

Query 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'

Would it be possible to combine these into 1 query or should I just combine the resulting arrays in PHP after doing both queries?

Answer 1

You can use UNION to merge two queries

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

you can use case when

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

You can combine the queries as

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'

Alternatively, UNION ALL might be used provided the columns categories.id and categories.top_cat are of the same data type, or able to be casted to the same data type.

Answer 4

Yes, UNION seems like a legit option. For your case it should not matter, since you are selecting from the same table, but as a side note you should know that when using UNION generally you need to have equal number of columns in your tables(that is if they were different).

Answer 5

You can do this with conditions in the WHERE clause:

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 
      );

Note:

• Table aliases make the query easier to write and to read.
• Assuming that the columns are numbers, don't compare the values to strings. Try to avoid type conversion issues.