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max(case when TIMESTAMPDIFF(Day, DATE(users.create_time), DATE(chat_records.event_time))= 1 then 1 else 0 end) yesterday,max的作用

Question

max(case when TIMESTAMPDIFF(Day, DATE(users.create_time), DATE(chat_records.event_time))= 1 then 1 else 0 end) yesterday,max的作用

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Solution

The SQL statement you provided is using the max function in combination with a case statement. Here's a step-by-step explanation:

  1. TIMESTAMPDIFF(Day, DATE(users.create_time), DATE(chat_records.event_time))= 1: This part of the statement is calculating the difference in days between the users.create_time and chat_records.event_time. If the difference is exactly 1 day, it returns true.

  2. case when ... then 1 else 0 end: This is a conditional statement. If the condition (TIMESTAMPDIFF(Day, DATE(users.create_time), DATE(chat_records.event_time))= 1) is true, it will return 1, otherwise, it will return 0.

  3. max(...): This is an aggregate function that returns the maximum value in a set of values. In this case, it will return the maximum value returned by the case statement.

So, the overall effect of this statement is to return 1 if there is at least one record where the difference between users.create_time and chat_records.event_time is exactly 1 day, and 0 otherwise. This could be used to check if there was any chat activity the day after each user's account was created.

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