howto - mysql count with unique entries only
Ever tried to count for a number of entries and figured out that the opposite/control query does not provide a fitting number? Maybe you forgot about the fact, that you have selected multiple entries per id. You can easily adapt your query to count only unique entries.
SELECT
COUNT(DISTINCT `my_table`.`my_column`)
FROM
`my_table` AS `t`
[
LEFT JOIN `my_second_table` AS `t2`
ON (`t`.`id` = `t2`.`my_table_id`
AND `t2`.`is_online` = 1)
]
[
WHERE
`t`.`my_column` = 'my_value`
]