Mysql-php statistics on the retention rate for seven days

The database has more than 50 thousand logon details. how can I check the table to calculate a high 7-day retention rate? The database has more than 50 thousand logon details. how can I check the table to calculate a high 7-day retention rate?

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The database has more than 50 thousand logon details. how can I check the table to calculate a high 7-day retention rate?

Register array key values in the format of datetime, month, and day;
Example:

$ Arr = ['1970-08-11 '=> [2016, 111,333, 23], // store the user ID registered on the current day '2017-08-12 '=> [],...]

Check the retention rate of users on November 11

Select count (*) from operation_log where client_id in () and login_time bewteen 1 and 2

Logon count in/login count in = retention rate

More than 50 thousand can be done, and direct PHP array processing does not consume too much memory.
You can use memcache or redis to store intermediate results.