php - Articles and Categories - The efficient way -

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i want add categories website - more complex sorting.

lets have article a. now, want a under main category a, on category c thats parent b.

i thought of doing this: article have main category field, , parent_ids

parent_ids every category item belong to. problem is: lets have: parent_ids: |1|5|2|7|.

now in category id - 7.

how select query like? 

select * categories parent_ids '%|7|%'

is efficient? there better idea?

never store multiple values in 1 column. column values in normalized database should atomic (single-valued).

the relationship between articles , categories many-to-many. means should have separate table, such article_category (article_id, category_id) containing 1 row each category each article in.


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