ls
pwd
cd
cd ..
mkdir //创建文件夹
touch dp.txt //创建文件
总结:
The command line is a text interface for the computer's operating system. To access the command line, we use the terminal.
A filesystem organizes a computer's files and directories into a tree structure. It starts with the root directory. Each parent directory can contain more child directories and files.
From...
多表查询
select albums.name,albums.year,artists.name from albums,artists;
SELECT * FROM albums JOIN artists ON albums.artist_id = artists.id;
SELECT * FROM albums LEFT JOIN artists ON albums.artist_id=artists.id;
select albums.name as 'Album',albums.year,artists.name as 'Artist' from albums join artists on albums.artist_id = artists.id where album...
计数
SELECT COUNT(*) FROM table_name; 总数
select count(*) from table_name where price=0; 价格等于0的数量
select price, count(*) from table_name group by price; //Count the number of apps at each price.
select price,count(*) from table_name where column > 20000 group by price;
SELECT SUM(column) FROM table_name;
SELECT category, SUM(column) FROM fake_...