Database Operations (v), aggregation functions, grouping

1, AVG (): average, null value does not participate in the calculation, participation in the calculation is required to use ISNULL ()

2, COUNT (): Number of records, null values do not participate in the calculation, participation in the calculation is required to use ISNULL ()

3, SUM (): record sum, null values do not participate in the calculation, participate in the calculation will need to use ISNULL (), in the sum of the operation if there is ' null ' in the operation, then the data value becomes NULL

4, Max (): The maximum value, null value does not participate in the calculation, participate in the calculation will need to use ISNULL ()

Min (): Minimum value, null value does not participate in calculation, participation in calculation is required to use ISNULL ()

5. GROUP BY: Group

Case when ... then ... else ... end: when .... At the time of establishment, otherwise, end

　　

Database Operations (v), aggregation functions, grouping