Question

I need to check which users added an item every day(‘to_date’ shows the day) since 20 days.

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**Table Structer**
# id         (int pk)
# user_id    (int fk)
# item_id    (int fk)   
# to_date    (datetime)    

Answer 1

This should work:

SELECT    user_id, COUNT(DISTINCT to_date) AS n
FROM      mytable
WHERE     to_date > DATE_SUB(CURRENT_DATE(), INTERVAL 20 DAY) 
GROUP BY  user_id
HAVING    n = 20

You may need to tweak the DATE_SUB clause to match your exact requirements.