Chapter 11 Connection query and group query

1. The principle of group query

        such as:

                 In a table, there are three semesters of people's test information. Now we want to query the number of people in each semester. At this time, we use select ---- from --- where will generate code redundancy, which is the garbage code we usually say. At this time, we will use the group by keyword.

2. Single column grouping query

     Syntax: eg:

               select column name, aggregate function from table name select count (*) as number, grade as grade  

               where constraints from Student

               group by column grouped by group

               order by column name

note:

        The select table can only contain:

                                          (1) Grouped columns

                                          (2) An expression that returns a value for each group, such as an aggregate function

3. Multi-column grouping query

       Syntax: eg:

                select column name 1, column name 2, aggregate function from table name select count (*) as number of people, grade as grade, sex as gender

                where constraints from Student

                group by Grouped column name 1, column name 2 group by grade, sex

                order by The column name to be sorted order by ----

      note:

                (1) Single column query does not require order by multi-column query try to add order by

                (2) Multi-column query grouping and sorting, the default is to sort the next one

4. Group screening (having):

             Syntax: eg:

                       select ----- from table name select count (*) as number of people, grade as grade

                       where ------                                                     from  Student

                       group by ------                                                 group  by  grade

                       having -------                                                    having count(*)>15

           

5. How to get a grade with a total number of more than 15 people, the address contains Shanghai

        select count (* ) as number of people, grade as grade from Student

        where  address  like'%上海%'

        group  by  grade

        having  count(*)>15

6. The difference between where and having:

      (1) Location:

                              where can be used in select, update, delete, and insert into values ​​(select * from table where ..) statements.

                              having can only be used in select statements

     (2) Execution order:

                                 

                                    where the search condition is applied before executing the statement to group

                                    Having the search condition is executed after the grouping condition

                                    That is, if and where are used together, where will be executed first, after having

     (3) The difference between clauses:

                                       The conditional expressions in the where clause can all follow, while some expressions in the where clause cannot be followed;

                                       Having clauses can use aggregate functions (sum, count, avg, max, and min), while where clauses cannot

     

     to sum up:

                   1. The WHERE clause is used to filter the rows generated by the operation specified in the FROM clause.
                   2. The GROUP BY clause is used to group the output of the WHERE clause.
                   3. The HAVING clause is used to filter rows from the grouped results