C language and SQL Server database technology

Fuzzy query and aggregate function query

One 、 use like Fuzzy query

1.like： image ....... equally

2. use like Fuzzy query can be done

3. Fuzzy query wildcard

                Underline _                          Match a character

                %                                  Match characters of any length

                [ ]                                  One character in the specified range ,[1-8] [a-z] [A-Z] [ Jin , package ]

                [^]                                  Any character that is not within the specified range

Two 、 Query in scope

1. Use between.....and

               a、between   20   and   30

               b、between    '1991-01-02'   and  '1992-03-05'

               c、 You can set the numerical range , You can also set the date range

               d、 Include the beginning , It also includes the end

               e、 Can pass not  between....and To set the reverse operation

2. Use in Query data in a limited range

               a、in（22,33,44）

               b、in（'aa','bb','cc'）

               c、not  in（22,33,44）

3、 ... and 、 Aggregate functions

1. Aggregate function is mainly used for statistical query

2. The aggregate function returns only one value , Therefore, it cannot be displayed directly with columns that return multiple rows

3.sum（）

4.avg（） averaging , Null values are ignored , for example ： Ask the average score of passing students

5.max（）/   min（） Find the maximum and minimum , It can be used to get the highest score and the lowest score

6.count（） Number of Statistics , Null values are ignored , You can count primary key columns or count（*） Realization

7. One sql Multiple aggregate functions can be used in queries , For example, query the highest and lowest scores of the class