mysql - How to get unique number of rows from the secondary table -

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create table pro_category (id primary key, product_id int, category_id int) insert pro_category (1, 1, 1) insert pro_category (1, 1, 2) insert pro_category (1, 2, 1) insert pro_category (1, 2, 1)

how unique number of rows secondary table (e.g. in above case there 2 product id's involved answer of 2).

using count(distinct)

select count(distinct product_id) pro_category

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