city
id name
1 Beijing
2 Shanghai
3 Tianjin
news Newslist
id name city
1 my 1
2 my 2
3 my 2
4 my 3
find out
how many messages are there in Beijing
how many messages are there in Shanghai
how many messages are there in Tianjin
SQL join is used to query data from two or more tables based on the relationship between the columns in those tables.
http://www.w3school.com.cn/sq.
hint: the id of the city table needs to correspond to the city of the news table
SQL group by is used to combine aggregate functions to group result sets according to one or more columns.
http://www.w3school.com.cn/sq.
hint: need to group by city
SQL count returns the number of rows selected in a given selection.
http://www.w3school.com.cn/sq.
hint: it's OK to use count (1), but the speed will be slightly faster
city table redundant one field record common number of entries num, but every time there is new or deleted data, you need to maintain the field. The advantage is that there is no need to make a correlation in statistics, and it can be obtained directly.
Reading websites often have today s page views, today s posts, today s comments and so on. Which statistical scheme is better? file + Cookie mode, the file on the server is emptied once in 24 hours. the dat file is accessed only once by the client...