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The SELECT statement is used to select data from a database.

The SQL SELECT Statement

The SELECT statement is used to select data from a database.
The result is stored in a result table, called the result-set.

SQL SELECT Syntax

SELECT column_name,column_name
FROM table_name;
and
SELECT * FROM table_name;

SELECT Column Example

The following SQL statement selects the
"CustomerName" and "City" columns from the "Customers" table:

Example

SELECT CustomerName,City FROM Customers;


SELECT * Example

The following SQL statement selects all the columns from the "Customers" table:

Example

SELECT * FROM Customers; 
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The SQL SELECT DISTINCT Statement

In a table, a column may contain many duplicate values; and sometimes you only want to list the different (distinct) values.
The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT Syntax

SELECT DISTINCT column_name,column_name
FROM table_name;

SELECT DISTINCT Example

The following SQL statement selects only the distinct values from the "City" columns from the "Customers" table:

Example

SELECT DISTINCT City FROM Customers; 
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The SQL WHERE Clause 

The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax

SELECT column_name,column_name
FROM table_name
WHERE column_name operator value;

WHERE Clause Example

The following SQL statement selects all the customers from the country "Mexico", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE Country='Mexico';

Text Fields vs. Numeric Fields

SQL requires single quotes around text values (most database systems will also allow double quotes).
However, numeric fields should not be enclosed in quotes:

Example

SELECT * FROM Customers
WHERE CustomerID=1;
Operator Description
= Equal
<> Not equal. Note: In some versions of SQL this operator may be written as !=
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
IN To specify multiple possible values for a column
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The SQL AND & OR Operators

The AND operator displays a record if both the first condition AND the second condition are true.
The OR operator displays a record if either the first condition OR the second condition is true.

AND Operator Example

The following SQL statement selects all customers from the country "Germany" AND the city "Berlin", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE Country='Germany'
AND City='Berlin'; 

OR Operator Example

The following SQL statement selects all customers from the city "Berlin" OR "München", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE City='Berlin'
OR City='München'; 

Combining AND & OR

You can also combine AND and OR (use parenthesis to form complex expressions).
The following SQL statement selects all customers from the country "Germany" AND the city must be equal to "Berlin" OR "München", in the "Customers" table:

Example

SELECT * FROM Customers
WHERE Country='Germany'
AND (City='Berlin' OR City='München');
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The SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set by one or more columns.
The ORDER BY keyword sorts the records in ascending order by default. To sort the records in a descending order, you can use the DESC keyword.

SQL ORDER BY Syntax

SELECT column_name,column_name
FROM table_name
ORDER BY column_name,column_name ASC|DESC;

ORDER BY Example

The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" column:

Example

SELECT * FROM Customers
ORDER BY Country; 

ORDER BY DESC Example

The following SQL statement selects all customers from the "Customers" table, sorted DESCENDING by the "Country" column:

Example

SELECT * FROM Customers
ORDER BY Country DESC;

ORDER BY Several Columns Example

The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" and the "CustomerName" column:

Example

SELECT * FROM Customers
ORDER BY Country,CustomerName;
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The SQL INSERT INTO Statement

The INSERT INTO statement is used to insert new records in a table.

SQL INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two forms.
The first form does not specify the column names where the data will be inserted, only their values:
INSERT INTO table_name
VALUES (value1,value2,value3,...);
The second form specifies both the column names and the values to be inserted:
INSERT INTO table_name (column1,column2,column3,...)
VALUES (value1,value2,value3,...);

INSERT INTO Example

Assume we wish to insert a new row in the "Customers" table.
We can use the following SQL statement:

Example

INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES ('Cardinal','Tom B. Erichsen','Skagen 21','Stavanger','4006','Norway'); 

Insert Data Only in Specified Columns

It is also possible to only insert data in specific columns.
The following SQL statement will insert a new row, but only insert data in the "CustomerName", "City", and "Country" columns (and the CustomerID field will of course also be updated automatically):

Example

INSERT INTO Customers (CustomerName, City, Country)
VALUES ('Cardinal', 'Stavanger', 'Norway'); 
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The SQL UPDATE Statement

The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax

UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;

Note Notice the WHERE clause in the SQL UPDATE statement!
The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!

SQL UPDATE Example

Assume we wish to update the customer "Alfreds Futterkiste" with a new contact person and city.
We use the following SQL statement:

Example

UPDATE Customers
SET ContactName='Alfred Schmidt', City='Hamburg'
WHERE CustomerName='Alfreds Futterkiste'; 

Update Warning!

Be careful when updating records. If we had omitted the WHERE clause, in the example above, like this:
UPDATE Customers
SET ContactName='Alfred Schmidt', City='Hamburg'; 
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The SQL DELETE Statement

The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax

DELETE FROM table_name
WHERE some_column=some_value;

Note Notice the WHERE clause in the SQL DELETE statement!
The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!

SQL DELETE Example

Assume we wish to delete the customer "Alfreds Futterkiste" from the "Customers" table.
We use the following SQL statement:

Example

DELETE FROM Customers
WHERE CustomerName='Alfreds Futterkiste' AND ContactName='Maria Anders'; 

Delete All Data

It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
DELETE FROM table_name;

or

DELETE * FROM table_name;
Note: Be very careful when deleting records. You cannot undo this statement!
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SQL Injection

An SQL Injection can destroy your database.

SQL in Web Pages

In the previous chapters, you have learned to retrieve (and update) database data, using SQL.
When SQL is used to display data on a web page, it is common to let web users input their own search values.
Since SQL statements are text only, it is easy, with a little piece of computer code, to dynamically change SQL statements to provide the user with selected data:

Server Code

txtUserId = getRequestString("UserId");
txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId;
The example above, creates a select statement by adding a variable (txtUserId) to a select string. The variable is fetched from the user input (Request) to the page.
The rest of this chapter describes the potential dangers of using user input in SQL statements.

SQL Injection

SQL injection is a technique where malicious users can inject SQL commands into an SQL statement, via web page input.
Injected SQL commands can alter SQL statement and compromise the security of a web application.

SQL Injection Based on 1=1 is Always True

Look at the example above, one more time.
Let's say that the original purpose of the code was to create an SQL statement to select a user with a given user id.
If there is nothing to prevent a user from entering "wrong" input, the user can enter some "smart" input like this:
UserId:

Server Result

SELECT * FROM Users WHERE UserId = 105 or 1=1
The SQL above is valid. It will return all rows from the table Users, since WHERE 1=1 is always true.
Does the example above seem dangerous? What if the Users table contains names and passwords?
The SQL statement above is much the same as this:
SELECT UserId, Name, Password FROM Users WHERE UserId = 105 or 1=1
A smart hacker might get access to all the user names and passwords in a database by simply inserting 105 or 1=1 into the input box.

SQL Injection Based on ""="" is Always True

Here is a common construction, used to verify user login to a web site:
User Name:

Password:

Server Code

uName = getRequestString("UserName");
uPass = getRequestString("UserPass");

sql = "SELECT * FROM Users WHERE Name ='" + uName + "' AND Pass ='" + uPass + "'"
A smart hacker might get access to user names and passwords in a database by simply inserting " or ""=" into the user name or password text box.
The code at the server will create a valid SQL statement like this:

Result

SELECT * FROM Users WHERE Name ="" or ""="" AND Pass ="" or ""=""
The result SQL is valid. It will return all rows from the table Users, since WHERE ""="" is always true.

SQL Injection Based on Batched SQL Statements 

Most databases support batched SQL statement, separated by semicolon.

Example

SELECT * FROM Users; DROP TABLE Suppliers
The SQL above will return all rows in the Users table, and then delete the table called Suppliers.
If we had the following server code:

Server Code

txtUserId = getRequestString("UserId");
txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId;
And the following input:
User id:

The code at the server would create a valid SQL statement like this:

Result

SELECT * FROM Users WHERE UserId = 105; DROP TABLE Suppliers


Parameters for Protection

Some web developers use a "blacklist" of words or characters to search for in SQL input, to prevent SQL injection attacks.
This is not a very good idea. Many of these words (like delete or drop) and characters (like semicolons and quotation marks), are used in common language, and should be allowed in many types of input.
(In fact it should be perfectly legal to input an SQL statement in a database field.)
The only proven way to protect a web site from SQL injection attacks, is to use SQL parameters.
SQL parameters are values that are added to an SQL query at execution time, in a controlled manner.

ASP.NET Razor Example

txtUserId = getRequestString("UserId");
txtSQL = "SELECT * FROM Users WHERE UserId = @0";
db.Execute(txtSQL,txtUserId);
Note that parameters are represented in the SQL statement by a @ marker.
The SQL engine checks each parameter to ensure that it is correct for its column and are treated literally, and not as part of the SQL to be executed.

Another Example

txtNam = getRequestString("CustomerName");
txtAdd = getRequestString("Address");
txtCit = getRequestString("City");
txtSQL = "INSERT INTO Customers (CustomerName,Address,City) Values(@0,@1,@2)";
db.Execute(txtSQL,txtNam,txtAdd,txtCit);

Note You have just learned to avoid SQL injection. One of the top website vulnerabilities.


Examples

The following examples shows how to build parameterized queries in some common web languages.
ASP.NET SELECT
txtUserId = getRequestString("UserId");
sql = "SELECT * FROM Customers WHERE CustomerId = @0";
command = new SqlCommand(sql);
command.Parameters.AddWithValue("@0",txtUserID);
command.ExecuteReader();
ASP.NET INSERT INTO
txtNam = getRequestString("CustomerName");
txtAdd = getRequestString("Address");
txtCit = getRequestString("City");
txtSQL = "INSERT INTO Customers (CustomerName,Address,City) Values(@0,@1,@2)";
command = new SqlCommand(txtSQL);
command.Parameters.AddWithValue("@0",txtNam);
command.Parameters.AddWithValue("@1",txtAdd);
command.Parameters.AddWithValue("@2",txtCit);
command.ExecuteNonQuery();
PHP INSERT INTO
$stmt = $dbh->prepare("INSERT INTO Customers (CustomerName,Address,City)
VALUES (:nam, :add, :cit)");
$stmt->bindParam(':nam', $txtNam);
$stmt->bindParam(':val', $txtAdd);
$stmt->bindParam(':cit', $txtCit);
$stmt->execute();
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The SQL SELECT TOP Clause

The SELECT TOP clause is used to specify the number of records to return.
The SELECT TOP clause can be very useful on large tables with thousands of records. Returning a large number of records can impact on performance.
Note: Not all database systems support the SELECT TOP clause.

SQL Server / MS Access Syntax

SELECT TOP number|percent column_name(s)
FROM table_name;


SQL SELECT TOP Equivalent in MySQL and Oracle

MySQL Syntax

SELECT column_name(s)
FROM table_name
LIMIT number;

Example

SELECT *
FROM Persons
LIMIT 5;

Oracle Syntax

SELECT column_name(s)
FROM table_name
WHERE ROWNUM <= number;

Example

SELECT *
FROM Persons
WHERE ROWNUM <=5;

SQL SELECT TOP Example

The following SQL statement selects the two first records from the "Customers" table:

Example

SELECT TOP 2 * FROM Customers; 

SQL SELECT TOP PERCENT Example

The following SQL statement selects the first 50% of the records from the "Customers" table:

Example

SELECT TOP 50 PERCENT * FROM Customers; 
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The SQL LIKE Operator

The LIKE operator is used to search for a specified pattern in a column.

SQL LIKE Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern;

SQL LIKE Operator Examples

The following SQL statement selects all customers with a City starting with the letter "s":

Example

SELECT * FROM Customers
WHERE City LIKE 's%'; 
Tip: The "%" sign is used to define wildcards (missing letters) both before and after the pattern. You will learn more about wildcards in the next chapter.
The following SQL statement selects all customers with a City ending with the letter "s":

Example

SELECT * FROM Customers
WHERE City LIKE '%s'; 
The following SQL statement selects all customers with a Country containing the pattern "land":

Example

SELECT * FROM Customers
WHERE Country LIKE '%land%';
 Using the NOT keyword allows you to select records that does NOT match the pattern.
The following SQL statement selects all customers with a Country NOT containing the pattern "land":

Example

SELECT * FROM Customers
WHERE Country NOT LIKE '%land%'; 
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SQL Wildcard Characters

In SQL, wildcard characters are used with the SQL LIKE operator.
SQL wildcards are used to search for data within a table.
With SQL, the wildcards are:
Wildcard Description
% A substitute for zero or more characters
_ A substitute for a single character
[charlist] Sets and ranges of characters to match
[^charlist]
or
[!charlist]

Using the SQL % Wildcard

The following SQL statement selects all customers with a City starting with "ber":

Example

SELECT * FROM Customers
WHERE City LIKE 'ber%'; 
The following SQL statement selects all customers with a City containing the pattern "es":

Example

SELECT * FROM Customers
WHERE City LIKE '%es%'; 

Using the SQL _ Wildcard

The following SQL statement selects all customers with a City starting with any character, followed by "erlin":

Example

SELECT * FROM Customers
WHERE City LIKE '_erlin'; 
The following SQL statement selects all customers with a City starting with "L", followed by any character, followed by "n", followed by any character, followed by "on":

Example

SELECT * FROM Customers
WHERE City LIKE 'L_n_on'; 

Using the SQL [charlist] Wildcard

The following SQL statement selects all customers with a City starting with "b", "s", or "p":

Example

SELECT * FROM Customers
WHERE City LIKE '[bsp]%'; 
The following SQL statement selects all customers with a City starting with "a", "b", or "c":

Example

SELECT * FROM Customers
WHERE City LIKE '[a-c]%'; 
The following SQL statement selects all customers with a City NOT starting with "b", "s", or "p":

Example

SELECT * FROM Customers
WHERE City LIKE '[!bsp]%'; 
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The IN Operator

The IN operator allows you to specify multiple values in a WHERE clause.

SQL IN Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1,value2,...);

IN Operator Example

The following SQL statement selects all customers with a City of "Paris" or "London":

Example

SELECT * FROM Customers
WHERE City IN ('Paris','London'); 
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The SQL BETWEEN Operator

The BETWEEN operator selects values within a range. The values can be numbers, text, or dates.

SQL BETWEEN Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name BETWEEN value1 AND value2;

BETWEEN Operator Example

The following SQL statement selects all products with a price BETWEEN 10 and 20:

Example

SELECT * FROM Products
WHERE Price BETWEEN 10 AND 20; 

NOT BETWEEN Operator Example

To display the products outside the range of the previous example, use NOT BETWEEN:

Example

SELECT * FROM Products
WHERE Price NOT BETWEEN 10 AND 20; 

BETWEEN Operator with IN Example

The following SQL statement selects all products with a price BETWEEN 10 and 20, but products with a CategoryID of 1,2, or 3 should not be displayed:

Example

SELECT * FROM Products
WHERE (Price BETWEEN 10 AND 20)
AND NOT CategoryID IN (1,2,3); 

BETWEEN Operator with Text Value Example

The following SQL statement selects all products with a ProductName beginning with any of the letter BETWEEN 'C' and 'M':

Example

SELECT * FROM Products
WHERE ProductName BETWEEN 'C' AND 'M'; 

NOT BETWEEN Operator with Text Value Example

The following SQL statement selects all products with a ProductName beginning with any of the letter NOT BETWEEN 'C' and 'M':

Example

SELECT * FROM Products
WHERE ProductName NOT BETWEEN 'C' AND 'M'; 

BETWEEN Operator with Date Value Example

The following SQL statement selects all orders with an OrderDate BETWEEN '04-July-1996' and '09-July-1996':

Example

SELECT * FROM Orders
WHERE OrderDate BETWEEN #07/04/1996# AND #07/09/1996#; 
Notice that the BETWEEN operator can produce different result in different databases!In some databases, BETWEEN selects fields that are between and excluding the test values.
In other databases, BETWEEN selects fields that are between and including the test values.
And in other databases, BETWEEN selects fields between the test values, including the first test value and excluding the last test value.
Therefore: Check how your database treats the BETWEEN operator!
__________________________________________________________________________________

SQL Aliases

SQL aliases are used to give a database table, or a column in a table, a temporary name.
Basically aliases are created to make column names more readable.

SQL Alias Syntax for Columns

SELECT column_name AS alias_name
FROM table_name;

SQL Alias Syntax for Tables

SELECT column_name(s)
FROM table_name AS alias_name; 

Alias Example for Table Columns

The following SQL statement specifies two aliases, one for the CustomerName column and one for the ContactName column. Tip: It require double quotation marks or square brackets if the column name contains spaces:

Example

SELECT CustomerName AS Customer, ContactName AS [Contact Person]
FROM Customers;
In the following SQL statement we combine four columns (Address, City, PostalCode, and Country) and create an alias named "Address":

Example

SELECT CustomerName, Address+', '+City+', '+PostalCode+', '+Country AS Address
FROM Customers;
Note: To get the SQL statement above to work in MySQL use the following:
SELECT CustomerName, CONCAT(Address,', ',City,', ',PostalCode,', ',Country) AS Address
FROM Customers;


Alias Example for Tables

The following SQL statement selects all the orders from the customer with CustomerID=4 (Around the Horn). We use the "Customers" and "Orders" tables, and give them the table aliases of "c" and "o" respectively (Here we have used aliases to make the SQL shorter):

Example

SELECT o.OrderID, o.OrderDate, c.CustomerName
FROM Customers AS c, Orders AS o
WHERE c.CustomerName="Around the Horn" AND c.CustomerID=o.CustomerID; 
The same SQL statement without aliases:

Example

SELECT Orders.OrderID, Orders.OrderDate, Customers.CustomerName
FROM Customers, Orders
WHERE Customers.CustomerName="Around the Horn" AND Customers.CustomerID=Orders.CustomerID; 
Aliases can be useful when:
  • There are more than one table involved in a query
  • Functions are used in the query
  • Column names are big or not very readable
  • Two or more columns are combined together
  • _____________________________________________________________________________
  • SQL JOIN

    An SQL JOIN clause is used to combine rows from two or more tables, based on a common field between them.
    The most common type of join is: SQL INNER JOIN (simple join). An SQL INNER JOIN return all rows from multiple tables where the join condition is met.

    if we run the following SQL statement (that contains an INNER JOIN):

    Example

    SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
    FROM Orders
    INNER JOIN Customers
    ON Orders.CustomerID=Customers.CustomerID; 

    Different SQL JOINs

    Before we continue with examples, we will list the types the different SQL JOINs you can use:
    • INNER JOIN: Returns all rows when there is at least one match in BOTH tables
    • LEFT JOIN: Return all rows from the left table, and the matched rows from the right table
    • RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table
    • FULL JOIN: Return all rows when there is a match in ONE of the tables
    • ________________________________________________________________________
    • SQL INNER JOIN Keyword

      The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables.

      SQL INNER JOIN Syntax

      SELECT column_name(s)
      FROM table1
      INNER JOIN table2
      ON table1.column_name=table2.column_name;
      or:
      SELECT column_name(s)
      FROM table1
      JOIN table2
      ON table1.column_name=table2.column_name;
      PS! INNER JOIN is the same as JOIN.

      SQL INNER JOIN Example

      The following SQL statement will return all customers with orders:

      Example

      SELECT Customers.CustomerName, Orders.OrderID
      FROM Customers
      INNER JOIN Orders
      ON Customers.CustomerID=Orders.CustomerID
      ORDER BY Customers.CustomerName;
      SQL INNER JOIN 

      Note: The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns. If there are rows in the "Customers" table that do not have matches in "Orders", these customers will NOT be listed.


      SQL LEFT JOIN Keyword

      The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match.

      SQL LEFT JOIN Syntax

      SELECT column_name(s)
      FROM table1
      LEFT JOIN table2
      ON table1.column_name=table2.column_name;
      or:
      SELECT column_name(s)
      FROM table1
      LEFT OUTER JOIN table2
      ON table1.column_name=table2.column_name;
      PS! In some databases LEFT JOIN is called LEFT OUTER JOIN.
      SQL LEFT JOIN 

      SQL LEFT JOIN Example

      The following SQL statement will return all customers, and any orders they might have:

      Example

      SELECT Customers.CustomerName, Orders.OrderID
      FROM Customers
      LEFT JOIN Orders
      ON Customers.CustomerID=Orders.CustomerID
      ORDER BY Customers.CustomerName; 
      Note: The LEFT JOIN keyword returns all the rows from the left table (Customers), even if there are no matches in the right table (Orders).

      SQL RIGHT JOIN Keyword

      The RIGHT JOIN keyword returns all rows from the right table (table2), with the matching rows in the left table (table1). The result is NULL in the left side when there is no match.

      SQL RIGHT JOIN Syntax

      SELECT column_name(s)
      FROM table1
      RIGHT JOIN table2
      ON table1.column_name=table2.column_name;
      or:
      SELECT column_name(s)
      FROM table1
      RIGHT OUTER JOIN table2
      ON table1.column_name=table2.column_name;
      PS! In some databases RIGHT JOIN is called RIGHT OUTER JOIN.
      SQL RIGHT JOIN 

      SQL RIGHT JOIN Example

      The following SQL statement will return all employees, and any orders they have placed:

      Example

      SELECT Orders.OrderID, Employees.FirstName
      FROM Orders
      RIGHT JOIN Employees
      ON Orders.EmployeeID=Employees.EmployeeID
      ORDER BY Orders.OrderID;
      Note: The RIGHT JOIN keyword returns all the rows from the right table (Employees), even if there are no matches in the left table (Orders).

      SQL FULL OUTER JOIN Keyword

      The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).
      The FULL OUTER JOIN keyword combines the result of both LEFT and RIGHT joins.

      SQL FULL OUTER JOIN Syntax

      SELECT column_name(s)
      FROM table1
      FULL OUTER JOIN table2
      ON table1.column_name=table2.column_name;
      SQL FULL OUTER JOIN 

      SQL FULL OUTER JOIN Example

      The following SQL statement selects all customers, and all orders:
      SELECT Customers.CustomerName, Orders.OrderID
      FROM Customers
      FULL OUTER JOIN Orders
      ON Customers.CustomerID=Orders.CustomerID
      ORDER BY Customers.CustomerName;





       

     

 



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