1596869400
What’s behind a simple SQL query?
Just like most kids these days do not seem to know where their meat comes from (hint: not growing on trees but could be from a lab) or why the sky is blue (hint: blue light has shorter, smaller waves), an alarming amount of developers do not (really) know where their data comes from and how it is processed & stored. This article should be a starting point into an engineer’s journey in understanding how that works. That is a very good thing regardless of what framework and no-code platform marketers are trying to teach you. Especially if data really is the new oil (apparently it has been for some years now), it might pay off to see how databases work.
Disclaimer: this is not a SQL tutorial
Without further adieu a simple SQL statement to fetch all users from a DB called users:
Disclaimer 2: the point is not to write fancy SQL statements but to showcase what is happening once you do
SELECT * FROM users;
So, how many of you actually know what is happening between pressing enter in your psql terminal and getting back that result?
Fundamentals
In order to understand this one has to start at the beginning — PostgresDB’s architecture.
At the end of the day it is a client/server application written in C++. A client is anyone who accesses the DB e.g. your psql terminal, your Lambda function, JDBC driver etc. and the server is the PGDB backend, which accepts your requests, stores the data etc.
Here is a basic architecture overview of PostgresDB that will be zoomed into throughout the article:
This is kind of oldschool but it still works :) Source: SlideShare
The Path of a Query
There is a pretty good overview of the “path of a query” in the depths of the official PostgresDB documentation. Here is what you should know:
1. Establish connection, transmit query & wait for results
PGDB can handle multiple connections concurrently (through the “postmaster”, see architecture) and for each connection it forks a new process (“postgres”, see architecture) to handle the requests (e.g. SQL statements) of that connection. In other words it is a simple “process per user” client/server model. The postmaster handles the initial connection and authentication and then hands that connection over to a new postgres process. Those processes communicate between each other with a shared memory and semaphores to ensure the overall data integrity even with concurrent connections (remember ACID?)
#sql #data #data-science #relational-databases #postgres #data analysis
What is GEEK
Buddha Community
1594369800
Introduction to Structured Query Language SQL pdf
SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.
Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:
1. The SQL order standard is genuinely intricate, and it isn’t handy to actualize the whole standard.
2. Every database seller needs an approach to separate its item from others.
Right now, contrasts are noted where fitting.
#programming books #beginning sql pdf #commands sql #download free sql full book pdf #introduction to sql pdf #introduction to sql ppt #introduction to sql #practical sql pdf #sql commands pdf with examples free download #sql commands #sql free bool download #sql guide #sql language #sql pdf #sql ppt #sql programming language #sql tutorial for beginners #sql tutorial pdf #sql #structured query language pdf #structured query language ppt #structured query language
1656151740
Test_cov_console: Flutter Console Coverage Test
Flutter Console Coverage Test
This small dart tools is used to generate Flutter Coverage Test report to console
How to install
Add a line like this to your package's pubspec.yaml (and run an implicit flutter pub get):
dev_dependencies:
test_cov_console: ^0.2.2
How to run
run the following command to make sure all flutter library is up-to-date
flutter pub get
Running "flutter pub get" in coverage... 0.5s
run the following command to generate lcov.info on coverage directory
flutter test --coverage
00:02 +1: All tests passed!
run the tool to generate report from lcov.info
flutter pub run test_cov_console
---------------------------------------------|---------|---------|---------|-------------------|
File |% Branch | % Funcs | % Lines | Uncovered Line #s |
---------------------------------------------|---------|---------|---------|-------------------|
lib/src/ | | | | |
print_cov.dart | 100.00 | 100.00 | 88.37 |...,149,205,206,207|
print_cov_constants.dart | 0.00 | 0.00 | 0.00 | no unit testing|
lib/ | | | | |
test_cov_console.dart | 0.00 | 0.00 | 0.00 | no unit testing|
---------------------------------------------|---------|---------|---------|-------------------|
All files with unit testing | 100.00 | 100.00 | 88.37 | |
---------------------------------------------|---------|---------|---------|-------------------|
Optional parameter
If not given a FILE, "coverage/lcov.info" will be used.
-f, --file=<FILE> The target lcov.info file to be reported
-e, --exclude=<STRING1,STRING2,...> A list of contains string for files without unit testing
to be excluded from report
-l, --line It will print Lines & Uncovered Lines only
Branch & Functions coverage percentage will not be printed
-i, --ignore It will not print any file without unit testing
-m, --multi Report from multiple lcov.info files
-c, --csv Output to CSV file
-o, --output=<CSV-FILE> Full path of output CSV file
If not given, "coverage/test_cov_console.csv" will be used
-t, --total Print only the total coverage
Note: it will ignore all other option (if any), except -m
-p, --pass=<MINIMUM> Print only the whether total coverage is passed MINIMUM value or not
If the value >= MINIMUM, it will print PASSED, otherwise FAILED
Note: it will ignore all other option (if any), except -m
-h, --help Show this help
example run the tool with parameters
flutter pub run test_cov_console --file=coverage/lcov.info --exclude=_constants,_mock
---------------------------------------------|---------|---------|---------|-------------------|
File |% Branch | % Funcs | % Lines | Uncovered Line #s |
---------------------------------------------|---------|---------|---------|-------------------|
lib/src/ | | | | |
print_cov.dart | 100.00 | 100.00 | 88.37 |...,149,205,206,207|
lib/ | | | | |
test_cov_console.dart | 0.00 | 0.00 | 0.00 | no unit testing|
---------------------------------------------|---------|---------|---------|-------------------|
All files with unit testing | 100.00 | 100.00 | 88.37 | |
---------------------------------------------|---------|---------|---------|-------------------|
report for multiple lcov.info files (-m, --multi)
It support to run for multiple lcov.info files with the followings directory structures:
1. No root module
<root>/<module_a>
<root>/<module_a>/coverage/lcov.info
<root>/<module_a>/lib/src
<root>/<module_b>
<root>/<module_b>/coverage/lcov.info
<root>/<module_b>/lib/src
...
2. With root module
<root>/coverage/lcov.info
<root>/lib/src
<root>/<module_a>
<root>/<module_a>/coverage/lcov.info
<root>/<module_a>/lib/src
<root>/<module_b>
<root>/<module_b>/coverage/lcov.info
<root>/<module_b>/lib/src
...
You must run test_cov_console on <root> dir, and the report would be grouped by module, here is
the sample output for directory structure 'with root module':
flutter pub run test_cov_console --file=coverage/lcov.info --exclude=_constants,_mock --multi
---------------------------------------------|---------|---------|---------|-------------------|
File |% Branch | % Funcs | % Lines | Uncovered Line #s |
---------------------------------------------|---------|---------|---------|-------------------|
lib/src/ | | | | |
print_cov.dart | 100.00 | 100.00 | 88.37 |...,149,205,206,207|
lib/ | | | | |
test_cov_console.dart | 0.00 | 0.00 | 0.00 | no unit testing|
---------------------------------------------|---------|---------|---------|-------------------|
All files with unit testing | 100.00 | 100.00 | 88.37 | |
---------------------------------------------|---------|---------|---------|-------------------|
---------------------------------------------|---------|---------|---------|-------------------|
File - module_a - |% Branch | % Funcs | % Lines | Uncovered Line #s |
---------------------------------------------|---------|---------|---------|-------------------|
lib/src/ | | | | |
print_cov.dart | 100.00 | 100.00 | 88.37 |...,149,205,206,207|
lib/ | | | | |
test_cov_console.dart | 0.00 | 0.00 | 0.00 | no unit testing|
---------------------------------------------|---------|---------|---------|-------------------|
All files with unit testing | 100.00 | 100.00 | 88.37 | |
---------------------------------------------|---------|---------|---------|-------------------|
---------------------------------------------|---------|---------|---------|-------------------|
File - module_b - |% Branch | % Funcs | % Lines | Uncovered Line #s |
---------------------------------------------|---------|---------|---------|-------------------|
lib/src/ | | | | |
print_cov.dart | 100.00 | 100.00 | 88.37 |...,149,205,206,207|
lib/ | | | | |
test_cov_console.dart | 0.00 | 0.00 | 0.00 | no unit testing|
---------------------------------------------|---------|---------|---------|-------------------|
All files with unit testing | 100.00 | 100.00 | 88.37 | |
---------------------------------------------|---------|---------|---------|-------------------|
Output to CSV file (-c, --csv, -o, --output)
flutter pub run test_cov_console -c --output=coverage/test_coverage.csv
#### sample CSV output file:
File,% Branch,% Funcs,% Lines,Uncovered Line #s
lib/,,,,
test_cov_console.dart,0.00,0.00,0.00,no unit testing
lib/src/,,,,
parser.dart,100.00,100.00,97.22,"97"
parser_constants.dart,100.00,100.00,100.00,""
print_cov.dart,100.00,100.00,82.91,"29,49,51,52,171,174,177,180,183,184,185,186,187,188,279,324,325,387,388,389,390,391,392,393,394,395,398"
print_cov_constants.dart,0.00,0.00,0.00,no unit testing
All files with unit testing,100.00,100.00,86.07,""Installing
Use this package as an executable
Install it
You can install the package from the command line:
dart pub global activate test_cov_consoleUse it
The package has the following executables:
$ test_cov_console
Use this package as a library
Depend on it
Run this command:
With Dart:
$ dart pub add test_cov_consoleWith Flutter:
$ flutter pub add test_cov_consoleThis will add a line like this to your package's pubspec.yaml (and run an implicit dart pub get):
dependencies:
test_cov_console: ^0.2.2Alternatively, your editor might support dart pub get or flutter pub get. Check the docs for your editor to learn more.
Import it
Now in your Dart code, you can use:
import 'package:test_cov_console/test_cov_console.dart';example/lib/main.dart
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
// This makes the visual density adapt to the platform that you run
// the app on. For desktop platforms, the controls will be smaller and
// closer together (more dense) than on mobile platforms.
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key? key, required this.title}) : super(key: key);
// This widget is the home page of your application. It is stateful, meaning
// that it has a State object (defined below) that contains fields that affect
// how it looks.
// This class is the configuration for the state. It holds the values (in this
// case the title) provided by the parent (in this case the App widget) and
// used by the build method of the State. Fields in a Widget subclass are
// always marked "final".
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
int _counter = 0;
void _incrementCounter() {
setState(() {
// This call to setState tells the Flutter framework that something has
// changed in this State, which causes it to rerun the build method below
// so that the display can reflect the updated values. If we changed
// _counter without calling setState(), then the build method would not be
// called again, and so nothing would appear to happen.
_counter++;
});
}
@override
Widget build(BuildContext context) {
// This method is rerun every time setState is called, for instance as done
// by the _incrementCounter method above.
//
// The Flutter framework has been optimized to make rerunning build methods
// fast, so that you can just rebuild anything that needs updating rather
// than having to individually change instances of widgets.
return Scaffold(
appBar: AppBar(
// Here we take the value from the MyHomePage object that was created by
// the App.build method, and use it to set our appbar title.
title: Text(widget.title),
),
body: Center(
// Center is a layout widget. It takes a single child and positions it
// in the middle of the parent.
child: Column(
// Column is also a layout widget. It takes a list of children and
// arranges them vertically. By default, it sizes itself to fit its
// children horizontally, and tries to be as tall as its parent.
//
// Invoke "debug painting" (press "p" in the console, choose the
// "Toggle Debug Paint" action from the Flutter Inspector in Android
// Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
// to see the wireframe for each widget.
//
// Column has various properties to control how it sizes itself and
// how it positions its children. Here we use mainAxisAlignment to
// center the children vertically; the main axis here is the vertical
// axis because Columns are vertical (the cross axis would be
// horizontal).
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'You have pushed the button this many times:',
),
Text(
'$_counter',
style: Theme.of(context).textTheme.headline4,
),
],
),
),
floatingActionButton: FloatingActionButton(
onPressed: _incrementCounter,
tooltip: 'Increment',
child: Icon(Icons.add),
), // This trailing comma makes auto-formatting nicer for build methods.
);
}
}Author: DigitalKatalis
Source Code: https://github.com/DigitalKatalis/test_cov_console
License: BSD-3-Clause license
1603760400
Introduction to Recursive CTE
This article will introduce the concept of SQL recursive. Recursive CTE is a really cool. We will see that it can often simplify our code, and avoid a cascade of SQL queries!
Why use a recursive CTE ?
The recursive queries are used to query hierarchical data. It avoids a cascade of SQL queries, you can only do one query to retrieve the hierarchical data.
What is recursive CTE ?
First, what is a CTE? A CTE (Common Table Expression) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. For example, you can use CTE when, in a query, you will use the same subquery more than once.
A recursive CTE is one having a subquery that refers to its own name!
Recursive CTE is defined in the SQL standard.
How to make a recursive CTE?
A recursive CTE has this structure:
- The WITH clause must begin with “WITH RECURSIVE”
- The recursive CTE subquery has two parts, separated by “UNION [ALL]” or “UNION DISTINCT”:
- The first part produces the initial row(s) for the CTE. This SELECT does not refer to the CTE name.
- The second part recurses by referring to the CTE name in its FROM clause.
Practice / Example
In this example, we use hierarchical data. Each row can have zero or one parent. And it parent can also have a parent etc.
Create table test (id integer, parent_id integer);
insert into test (id, parent_id) values (1, null);
insert into test (id, parent_id) values (11, 1);
insert into test (id, parent_id) values (111, 11);
insert into test (id, parent_id) values (112, 11);
insert into test (id, parent_id) values (12, 1);
insert into test (id, parent_id) values (121, 12);
For example, the row with id 111 has as ancestors: 11 and 1.
Before knowing the recursive CTE, I was doing several queries to get all the ancestors of a row.
For example, to retrieve all the ancestors of the row with id 111.
While (has parent)
Select id, parent_id from test where id = X
With recursive CTE, we can retrieve all ancestors of a row with only one SQL query :)
WITH RECURSIVE cte_test AS (
SELECT id, parent_id FROM test WHERE id = 111
UNION
SELECT test.id, test.parent_id FROM test JOIN cte_test ON cte_test.id = test.parent_id
) SELECT * FROM cte_test
Explanations:
- “WITH RECURSIVE”:
It indicates we will make recursive
- “SELECT id, parent_id FROM test WHERE id = 111”:
It is the initial query.
- “UNION … JOIN cte_test” :
It is the recursive expression! We make a jointure with the current CTE!
Replay this example here
#sql #database #sql-server #sql-injection #writing-sql-queries #sql-beginner-tips #better-sql-querying-tips #sql-top-story
1596441660
Welcome Back the T-SQL Debugger with SQL Complete – SQL Debugger
When you develop large chunks of T-SQL code with the help of the SQL Server Management Studio tool, it is essential to test the “Live” behavior of your code by making sure that each small piece of code works fine and being able to allocate any error message that may cause a failure within that code.
The easiest way to perform that would be to use the T-SQL debugger feature, which used to be built-in over the SQL Server Management Studio tool. But since the T-SQL debugger feature was removed completely from SQL Server Management Studio 18 and later editions, we need a replacement for that feature. This is because we cannot keep using the old versions of SSMS just to support the T-SQL Debugger feature without “enjoying” the new features and bug fixes that are released in the new SSMS versions.
If you plan to wait for SSMS to bring back the T-SQL Debugger feature, vote in the Put Debugger back into SSMS 18 to ask Microsoft to reintroduce it.
As for me, I searched for an alternative tool for a T-SQL Debugger SSMS built-in feature and found that Devart company rolled out a new T-SQL Debugger feature to version 6.4 of SQL – Complete tool. SQL Complete is an add-in for Visual Studio and SSMS that offers scripts autocompletion capabilities, which help develop and debug your SQL database project.
The SQL Debugger feature of SQL Complete allows you to check the execution of your scripts, procedures, functions, and triggers step by step by adding breakpoints to the lines where you plan to start, suspend, evaluate, step through, and then to continue the execution of your script.
You can download SQL Complete from the dbForge Download page and install it on your machine using a straight-forward installation wizard. The wizard will ask you to specify the installation path for the SQL Complete tool and the versions of SSMS and Visual Studio that you plan to install the SQL Complete on, as an add-in, from the versions that are installed on your machine, as shown below:
Once SQL Complete is fully installed on your machine, the dbForge SQL Complete installation wizard will notify you of whether the installation was completed successfully or the wizard faced any specific issue that you can troubleshoot and fix easily. If there are no issues, the wizard will provide you with an option to open the SSMS tool and start using the SQL Complete tool, as displayed below:
When you open SSMS, you will see a new “Debug” tools menu, under which you can navigate the SQL Debugger feature options. Besides, you will see a list of icons that will be used to control the debug mode of the T-SQL query at the leftmost side of the SSMS tool. If you cannot see the list, you can go to View -> Toolbars -> Debugger to make these icons visible.
During the debugging session, the SQL Debugger icons will be as follows:
The functionality of these icons within the SQL Debugger can be summarized as:
- Adding Breakpoints to control the execution pause of the T-SQL script at a specific statement allows you to check the debugging information of the T-SQL statements such as the values for the parameters and the variables.
- Step Into is “navigate” through the script statements one by one, allowing you to check how each statement behaves.
- Step Over is “execute” a specific stored procedure if you are sure that it contains no error.
- Step Out is “return” from the stored procedure, function, or trigger to the main debugging window.
- Continue executing the script until reaching the next breakpoint.
- Stop Debugging is “terminate” the debugging session.
- Restart “stop and start” the current debugging session.
#sql server #sql #sql debugger #sql server #sql server stored procedure #ssms #t-sql queries
1618936320
How to Write Simple SQL Queries from a Blank Database
This article explains writing simple SQL queries from the most basic ones and gradually improving the script to solve some mathematical and date-related problems. Additionally, we are going to clarify the concepts surrounding SQL queries.
Although this article is primarily for beginners, it contains hints that will be helpful for any experience level.
#sql server #sql query #t-sql queries #sql