sql计算留存率

SELECT first_day, sum(case when by_day = 0 then 1 else 0 end) day_0, sum(case when by_day = 1 then 1 else 0 end) day_1, sum(case when by_day = 2 then 1 else 0 end) day_2, sum(case when by_day = 3 then

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