Some notable ones:
This is particularly important in SQL: Nested set model in SQL, as it is an efficient way to transverse trees there, since querying parents every time would require multiple disk accesses.
The ASCII art visualizations from stackoverflow.com/questions/192220/what-is-the-most-efficient-elegant-way-to-parse-a-flat-table-into-a-tree/194031#194031 are worth reproducing.
As the sets:
__________________________________________________________________________
| Root 1 |
| ________________________________ ________________________________ |
| | Child 1.1 | | Child 1.2 | |
| | ___________ ___________ | | ___________ ___________ | |
| | | C 1.1.1 | | C 1.1.2 | | | | C 1.2.1 | | C 1.2.2 | | |
1 2 3___________4 5___________6 7 8 9___________10 11__________12 13 14
| |________________________________| |________________________________| |
|__________________________________________________________________________|Consider the following nested set:
0, 8, root
1, 7, mathematics
2, 3, geometry
3, 6, calculus
4, 5, derivative
5, 6, integral
6, 7, algebra
7, 8, physicsEach transaction isolation level specifies what can or cannot happen when two queries are being run in parallel, i.e.: the memory semantics of the system.
Remember that queries can affects thousands of rows, and database systems like PostgreSQL can run multiple such queries at the same time.
Implementation specifics:
Login without password: askubuntu.com/questions/915585/how-to-login-mysql-shell-when-mysql-have-no-passwordworks on Ubuntu 20.10.
sudo mysqlCreate user for further logins without
sudo askubuntu.com/questions/915585/how-to-login-mysql-shell-when-mysql-have-no-password/1325689#1325689:sudo mysql -e "CREATE USER $USER"Run command from CLI stackoverflow.com/questions/1602904/how-do-you-run-a-single-query-through-mysql-from-the-command-line
mysql -e 'SHOW DATABASES'Create test user with password:and login as that user:Login with password given on the command line:The
sudo mysql -e 'CREATE USER user0 IDENTIFIED WITH mysql_native_password BY "a"'
sudo mysql -e 'GRANT ALL PRIVILEGES ON database_name.* TO "user0"'mysql -u user0 -pmysql -u user0 -pmypasswordIDENTIFIED WITH mysql_native_password part is to overcome "Client does not support authentication protocol requested by server" when connecting from Node.js.List users:
sudo mysql -e 'SELECT * FROM mysql.user'View permissions for each user on each DB: serverfault.com/questions/263868/how-to-know-all-the-users-that-can-access-a-database-mysql
sudo mysql -e 'SELECT * FROM mysql.db'List databases:
sudo mysql -e 'SHOW DATABASES'Create database:
sudo mysql -e 'CREATE DATABASE mydb0'Destroy database:
sudo mysql -e 'DROP DATABASE mydb0'PostgreSQL feels good.
Its feature set is insanely large! Just look at stuff like: stackoverflow.com/questions/1986491/sql-split-string-by-space-into-table-in-postgresql/1993058#1993058
If Oracle is the Microsoft of database, Postgres is the Linux, and MySQL (or more precisely MariaDB) is the FreeBSD (i.e. the one that got delayed by legal issues). Except that their software licenses were accidentally swapped.
The only problem with Postgres is its name. PostgreSQL is so unpronounceable and so untypeable that you should just call it "Postgres" like everyone else.
The minimalism, serverlessness/lack of temporary caches/lack of permission management, Hipp's religious obsession with efficiency, the use of their own pure Fossil version control[ref]. Wait, scrap that last one. Pure beauty!
Official Git mirror: github.com/sqlite/sqlite
Create a table
sqlite3 db.sqlite3 "
CREATE TABLE 'IntegerNames' (int0 INT, char0 CHAR(16));
INSERT INTO 'IntegerNames' (int0, char0) VALUES (2, 'two'), (3, 'three'), (5, 'five'), (7, 'seven');
"List tables:output:
sqlite3 db.sqlite3 '.tables'IntegerNamesShow schema of a table:outputs the query that would generate that table:
sqlite3 db.sqlite3 '.schema IntegerNames'CREATE TABLE IF NOT EXISTS 'IntegerNames' (int0 INT, char0 CHAR(16)); There are unlisted articles, also show them or only show them.