In the following query, we list all lastname held by 2 or more employees.
SELECT lastname, COUNT(*) c FROM employees GROUP BY lastname HAVING c > 1;
If you don't want a value to unique, you should add a unique constraint to the column. However, you would from time to time have to deal with a badly designed database.
/bin/rm: cannot execute [Argument list too long]
rm can only handle a certain number of arguments. If you run following command in a directory of 500 json files, the argument list is 500
$ rm -f *.json
Use find or find coupled with xargs as follows:
$ find . -name "*.json" -print0 | xargs -0 rm
or
If you don't already have MySQL installed, install it with the following commands (mac only):
$ brew info mysql
$ brew install mysql
$ brew tap homebrew/services
Run the following commands to start or stop mysql. Start MySQL before proceeding:
$ brew services stop mysql
$ brew services start mysql
To see available sevices:
$ brew services list
To check MySQL version:
$ mysql -V
Set root password
These instructions are for Mac OS:
First install homebrew. Never use sudo for brew.
$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
$ which brew
/usr/local/bin/brew
Install Python3. The installation comes with pip3.
$ brew install python3
$ which pip3
/usr/local/bin/pip3
Install cx_Oracle, the library that allows python3 to communicate with Oracle database.
Node uses the fs library to provide rich and powerful file management functionality. First let's a directory and some files:
$ mkdir data; cd data
$ touch provinces.txt capitals.txt
Add the following text to provinces.txt
Alberta
British Columbia
Manitoba
New Brunswick
Newfoundland and Labrador
Nova Scotia
Ontario
Prince Edward Island
Quebec
Saskatchewan
Add the following to capitals.txt
The child_process module allows you to communicate with other applications on your system by executing external processes in your environment. There are four kinds of child processes; exec(), spawn(), execFile(), fork(). I will be covering exec() and spawn() here. If a child process will execute once, use exec() function. For example the "cal" command on terminal prints the current month's calendar in the terminal. The command is executed once, prints a result and that's it. If a child process would be running for a long time and printing output several times, use spawn() function.
Node.js is an event-driven system. When you are performing different tasks, different events are emitted. You need to understand those events and how to use them. Events are defined by different modules. For example process.on() is an event. You can also create and use your own events. See example:
var events = require('events');
var eventEmitter = new events.EventEmitter();
eventEmitter.on('bankrupt', function() {
console.log('You are bankrupt');
});
var networth = -20000;
if (networth < 0) {
eventEmitter.emit('bankrupt');
}
output:
A module encapsulates relate code, such as code for handling files, into a single unit of code. Node ships with many useful modules. These are called built-in modules. Programmers can create new modules. These are called custom modules.
Following is a list of some built-in modules:
Node support both synchronous and asynchronous communication. What is the difference? Synchronous communication is sequential communication. You can only run one job at a time and all jobs need to wait for their turn to run. In asynchronous communication, the jobs can run in parallel depending on available resources. Following is a simplified example.
The following code creates a server and assign a port.
var http = require('http');
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end('Your node server is running');
}).listen(8282);
Simply run the code and open http://localhost:8282 in your browser.