SQL practice

1. The DATEDIFF() function returns the time between two dates.

The syntax of the DATEDIFF() function in MySQL is DATEDIFF(date1,date2), which returns the number of days between two dates

2. distinct deduplicate data

select distinct name, id from A

The result after execution is as follows:

Actually, the deduplication is based on the two fields of name and id. This method is supported by both Access and SQL Server.

3. Aggregated data

The execution process of the sql statement is: from-->where-->group by -->having --- >order by --> select; The aggregation function is performed for the result set, but where is the query resultThe result set is filtered before, so when the aggregation function is used together with where, an error will occur. Having is for filtering the result set, so the aggregation function is matched with group by, and having is used instead of where. Having generally follows group by.