Cayla  Erdman

Cayla Erdman

1598446500

The Ultimate SQL Guide for Beginners in 2020.

SQL is a standard language for storing, manipulating, and retrieving data in databases. In this article, I’ll teach you the very basic fundamentals of the SQL language and hope you will be able to write your own database queries at the end.

What does SQL Mean?

SQL stands for Structured Query Language and lets you access and manipulate databases.

Syntax

Most of the actions you need to perform on a database are done with SQL statements. The following SQL statement selects all the records in the “Users” table:

SELECT * FROM Users;

Select

The select statement is used to retrieve data from a database. The requested data is returned in a results table.

SELECT column1 FROM table_name;

Select Distinct

The Select Distinct statement is used to return only distinct (different) values.

SELECT DISTINCT * FROM table_name;

Count

The following SQL statement lists the number of different customer countries:

SELECT COUNT(DISTINCT Country) FROM Customers;

Where

The Where clause is used to filter records.

SELECT column1
FROM table_name
WHERE condition;

For example:

SELECT * FROM Users
WHERE Country='Netherlands';

AND, OR and NOT Operators

The Where clause can be combined with AND, OR, and NOT operators. The AND and OR operators are used to filter records based on more than one condition:

  • The AND operator displays a record if all the conditions separated by AND are TRUE.
  • The OR operator displays a record if any of the conditions separated by OR is TRUE.

The NOT operator displays a record if the condition(s) is NOT TRUE.

AND

SELECT column1, column2, ...
FROM table_name
WHERE condition1 AND condition2 AND condition3 ...;

OR

SELECT column1, column2, ...
FROM table_name
WHERE condition1 OR condition2 OR condition3 ...;

NOT

SELECT column1, column2, ...
FROM table_name
WHERE NOT condition;

#tech #guides-and-tutorials #sql #beginners-guide #programming

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The Ultimate SQL Guide for Beginners in 2020.
Cayla  Erdman

Cayla Erdman

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

Brain  Crist

Brain Crist

1594753020

Citrix Bugs Allow Unauthenticated Code Injection, Data Theft

Multiple vulnerabilities in the Citrix Application Delivery Controller (ADC) and Gateway would allow code injection, information disclosure and denial of service, the networking vendor announced Tuesday. Four of the bugs are exploitable by an unauthenticated, remote attacker.

The Citrix products (formerly known as NetScaler ADC and Gateway) are used for application-aware traffic management and secure remote access, respectively, and are installed in at least 80,000 companies in 158 countries, according to a December assessment from Positive Technologies.

Other flaws announced Tuesday also affect Citrix SD-WAN WANOP appliances, models 4000-WO, 4100-WO, 5000-WO and 5100-WO.

Attacks on the management interface of the products could result in system compromise by an unauthenticated user on the management network; or system compromise through cross-site scripting (XSS). Attackers could also create a download link for the device which, if downloaded and then executed by an unauthenticated user on the management network, could result in the compromise of a local computer.

“Customers who have configured their systems in accordance with Citrix recommendations [i.e., to have this interface separated from the network and protected by a firewall] have significantly reduced their risk from attacks to the management interface,” according to the vendor.

Threat actors could also mount attacks on Virtual IPs (VIPs). VIPs, among other things, are used to provide users with a unique IP address for communicating with network resources for applications that do not allow multiple connections or users from the same IP address.

The VIP attacks include denial of service against either the Gateway or Authentication virtual servers by an unauthenticated user; or remote port scanning of the internal network by an authenticated Citrix Gateway user.

“Attackers can only discern whether a TLS connection is possible with the port and cannot communicate further with the end devices,” according to the critical Citrix advisory. “Customers who have not enabled either the Gateway or Authentication virtual servers are not at risk from attacks that are applicable to those servers. Other virtual servers e.g. load balancing and content switching virtual servers are not affected by these issues.”

A final vulnerability has been found in Citrix Gateway Plug-in for Linux that would allow a local logged-on user of a Linux system with that plug-in installed to elevate their privileges to an administrator account on that computer, the company said.

#vulnerabilities #adc #citrix #code injection #critical advisory #cve-2020-8187 #cve-2020-8190 #cve-2020-8191 #cve-2020-8193 #cve-2020-8194 #cve-2020-8195 #cve-2020-8196 #cve-2020-8197 #cve-2020-8198 #cve-2020-8199 #denial of service #gateway #information disclosure #patches #security advisory #security bugs

Cayla  Erdman

Cayla Erdman

1598446500

The Ultimate SQL Guide for Beginners in 2020.

SQL is a standard language for storing, manipulating, and retrieving data in databases. In this article, I’ll teach you the very basic fundamentals of the SQL language and hope you will be able to write your own database queries at the end.

What does SQL Mean?

SQL stands for Structured Query Language and lets you access and manipulate databases.

Syntax

Most of the actions you need to perform on a database are done with SQL statements. The following SQL statement selects all the records in the “Users” table:

SELECT * FROM Users;

Select

The select statement is used to retrieve data from a database. The requested data is returned in a results table.

SELECT column1 FROM table_name;

Select Distinct

The Select Distinct statement is used to return only distinct (different) values.

SELECT DISTINCT * FROM table_name;

Count

The following SQL statement lists the number of different customer countries:

SELECT COUNT(DISTINCT Country) FROM Customers;

Where

The Where clause is used to filter records.

SELECT column1
FROM table_name
WHERE condition;

For example:

SELECT * FROM Users
WHERE Country='Netherlands';

AND, OR and NOT Operators

The Where clause can be combined with AND, OR, and NOT operators. The AND and OR operators are used to filter records based on more than one condition:

  • The AND operator displays a record if all the conditions separated by AND are TRUE.
  • The OR operator displays a record if any of the conditions separated by OR is TRUE.

The NOT operator displays a record if the condition(s) is NOT TRUE.

AND

SELECT column1, column2, ...
FROM table_name
WHERE condition1 AND condition2 AND condition3 ...;

OR

SELECT column1, column2, ...
FROM table_name
WHERE condition1 OR condition2 OR condition3 ...;

NOT

SELECT column1, column2, ...
FROM table_name
WHERE NOT condition;

#tech #guides-and-tutorials #sql #beginners-guide #programming

Cayla  Erdman

Cayla Erdman

1596448980

The Easy Guide on How to Use Subqueries in SQL Server

Let’s say the chief credit and collections officer asks you to list down the names of people, their unpaid balances per month, and the current running balance and wants you to import this data array into Excel. The purpose is to analyze the data and come up with an offer making payments lighter to mitigate the effects of the COVID19 pandemic.

Do you opt to use a query and a nested subquery or a join? What decision will you make?

SQL Subqueries – What Are They?

Before we do a deep dive into syntax, performance impact, and caveats, why not define a subquery first?

In the simplest terms, a subquery is a query within a query. While a query that embodies a subquery is the outer query, we refer to a subquery as the inner query or inner select. And parentheses enclose a subquery similar to the structure below:

SELECT 
 col1
,col2
,(subquery) as col3
FROM table1
[JOIN table2 ON table1.col1 = table2.col2]
WHERE col1 <operator> (subquery)

We are going to look upon the following points in this post:

  • SQL subquery syntax depending on different subquery types and operators.
  • When and in what sort of statements one can use a subquery.
  • Performance implications vs. JOINs.
  • Common caveats when using SQL subqueries.

As is customary, we provide examples and illustrations to enhance understanding. But bear in mind that the main focus of this post is on subqueries in SQL Server.

Now, let’s get started.

Make SQL Subqueries That Are Self-Contained or Correlated

For one thing, subqueries are categorized based on their dependency on the outer query.

Let me describe what a self-contained subquery is.

Self-contained subqueries (or sometimes referred to as non-correlated or simple subqueries) are independent of the tables in the outer query. Let me illustrate this:

-- Get sales orders of customers from Southwest United States 
-- (TerritoryID = 4)

USE [AdventureWorks]
GO
SELECT CustomerID, SalesOrderID
FROM Sales.SalesOrderHeader
WHERE CustomerID IN (SELECT [CustomerID]
                     FROM [AdventureWorks].[Sales].[Customer]
                     WHERE TerritoryID = 4)

As demonstrated in the above code, the subquery (enclosed in parentheses below) has no references to any column in the outer query. Additionally, you can highlight the subquery in SQL Server Management Studio and execute it without getting any runtime errors.

Which, in turn, leads to easier debugging of self-contained subqueries.

The next thing to consider is correlated subqueries. Compared to its self-contained counterpart, this one has at least one column being referenced from the outer query. To clarify, I will provide an example:

USE [AdventureWorks]
GO
SELECT DISTINCT a.LastName, a.FirstName, b.BusinessEntityID
FROM Person.Person AS p
JOIN HumanResources.Employee AS e ON p.BusinessEntityID = e.BusinessEntityID
WHERE 1262000.00 IN
    (SELECT [SalesQuota]
    FROM Sales.SalesPersonQuotaHistory spq
    WHERE p.BusinessEntityID = spq.BusinessEntityID)

Were you attentive enough to notice the reference to BusinessEntityID from the Person table? Well done!

Once a column from the outer query is referenced in the subquery, it becomes a correlated subquery. One more point to consider: if you highlight a subquery and execute it, an error will occur.

And yes, you are absolutely right: this makes correlated subqueries pretty harder to debug.

To make debugging possible, follow these steps:

  • isolate the subquery.
  • replace the reference to the outer query with a constant value.

Isolating the subquery for debugging will make it look like this:

SELECT [SalesQuota]
    FROM Sales.SalesPersonQuotaHistory spq
    WHERE spq.BusinessEntityID = <constant value>

Now, let’s dig a little deeper into the output of subqueries.

Make SQL Subqueries With 3 Possible Returned Values

Well, first, let’s think of what returned values can we expect from SQL subqueries.

In fact, there are 3 possible outcomes:

  • A single value
  • Multiple values
  • Whole tables

Single Value

Let’s start with single-valued output. This type of subquery can appear anywhere in the outer query where an expression is expected, like the WHERE clause.

-- Output a single value which is the maximum or last TransactionID
USE [AdventureWorks]
GO
SELECT TransactionID, ProductID, TransactionDate, Quantity
FROM Production.TransactionHistory
WHERE TransactionID = (SELECT MAX(t.TransactionID) 
                       FROM Production.TransactionHistory t)

When you use a MAX() function, you retrieve a single value. That’s exactly what happened to our subquery above. Using the equal (=) operator tells SQL Server that you expect a single value. Another thing: if the subquery returns multiple values using the equals (=) operator, you get an error, similar to the one below:

Msg 512, Level 16, State 1, Line 20
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Multiple Values

Next, we examine the multi-valued output. This kind of subquery returns a list of values with a single column. Additionally, operators like IN and NOT IN will expect one or more values.

-- Output multiple values which is a list of customers with lastnames that --- start with 'I'

USE [AdventureWorks]
GO
SELECT [SalesOrderID], [OrderDate], [ShipDate], [CustomerID]
FROM Sales.SalesOrderHeader
WHERE [CustomerID] IN (SELECT c.[CustomerID] FROM Sales.Customer c
INNER JOIN Person.Person p ON c.PersonID = p.BusinessEntityID
WHERE p.lastname LIKE N'I%' AND p.PersonType='SC')

Whole Table Values

And last but not least, why not delve into whole table outputs.

-- Output a table of values based on sales orders
USE [AdventureWorks]
GO
SELECT [ShipYear],
COUNT(DISTINCT [CustomerID]) AS CustomerCount
FROM (SELECT YEAR([ShipDate]) AS [ShipYear], [CustomerID] 
      FROM Sales.SalesOrderHeader) AS Shipments
GROUP BY [ShipYear]
ORDER BY [ShipYear]

Have you noticed the FROM clause?

Instead of using a table, it used a subquery. This is called a derived table or a table subquery.

And now, let me present you some ground rules when using this sort of query:

  • All columns in the subquery should have unique names. Much like a physical table, a derived table should have unique column names.
  • ORDER BY is not allowed unless TOP is also specified. That’s because the derived table represents a relational table where rows have no defined order.

In this case, a derived table has the benefits of a physical table. That’s why in our example, we can use COUNT() in one of the columns of the derived table.

That’s about all regarding subquery outputs. But before we get any further, you may have noticed that the logic behind the example for multiple values and others as well can also be done using a JOIN.

-- Output multiple values which is a list of customers with lastnames that start with 'I'
USE [AdventureWorks]
GO
SELECT o.[SalesOrderID], o.[OrderDate], o.[ShipDate], o.[CustomerID]
FROM Sales.SalesOrderHeader o
INNER JOIN Sales.Customer c on o.CustomerID = c.CustomerID
INNER JOIN Person.Person p ON c.PersonID = p.BusinessEntityID
WHERE p.LastName LIKE N'I%' AND p.PersonType = 'SC'

In fact, the output will be the same. But which one performs better?

Before we get into that, let me tell you that I have dedicated a section to this hot topic. We’ll examine it with complete execution plans and have a look at illustrations.

So, bear with me for a moment. Let’s discuss another way to place your subqueries.

#sql server #sql query #sql server #sql subqueries #t-sql statements #sql

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