SQL foundation 9 [grouping data]

1. The data packet
   Using grouping, you can divide data into multiple logical groups , And perform aggregation calculation for each group .

2. Create grouping
   Grouping is using select Of the statement group by Clause .group by It can be considered as putting the data in a table , According to the value of the grouping field , It is divided into several different sub tables
   
   The meaning of the above sentence ： Counter table tb_order Press order_status Grouping , Return to a different order_status, And their number . give the result as follows .

   
   *group by Clause can contain any number of columns , Thus, groups can be nested , More detailed data grouping .
   * If in group by Group nested in Clause , The data will be summarized on the last specified group .
   *group by Each column listed in clause must be a number of search columns or valid expressions ., If in select Using expressions in , Must be in group by Clause specifies the same expression .
   * In addition to aggregate calculation statements ,select Each column in the statement must be in group by Given in Clause .
   * If the grouping column null, be null Will be returned as a group .
   *group by Clause must appear in where Before clause ,order by After Clause .

3. Filter grouping
   Use having Filter groups , All types of where Clause can be used having Instead of , The only difference is ,where Filter line , and having Filter grouping .
   
   
   *where Filter before data grouping ,having Filter after data grouping ,where Excluded rows are not included in the group .
   Enter the following example , Filter out receiver_address For the record of Zhongteng building .
   
   

4. Grouping and sorting
   order by Sort the resulting output ,group by Group lines , But the output may not be in the order of grouping .

5. select Clause order