SQL INNER JOIN Keyword

SQL LEFT JOIN Keyword :

The LEFT JOIN keyword returns all rows from the left table (table_name1), even if there are no matches in the right table (table_name2).

SQL LEFT JOIN Syntax :

SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name=table_name2.column_name

PS: In some databases LEFT JOIN is called LEFT OUTER JOIN.
----------------------------------

SQL LEFT JOIN Example :
The "Persons" table:

P_Id LastName FirstName Address City
------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id
------------------
1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Now we want to list all the persons and their orders - if any, from the tables above.

We use the following SELECT statement:

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
LEFT JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

The result-set will look like this:

LastName FirstName OrderNo
--------------------------
Hansen Ola 22456
Hansen Ola 24562
Pettersen Kari 77895
Pettersen Kari 44678
Svendson Tove

The LEFT JOIN keyword returns all the rows from the left table (Persons), even if there are no matches in the right table (Orders).

SQL INNER JOIN Keyword

SQL INNER JOIN Keyword :

The INNER JOIN keyword return rows when there is at least one match in both tables.

SQL INNER JOIN Syntax :

SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2
ON table_name1.column_name=table_name2.column_name

PS: INNER JOIN is the same as JOIN.
---------------------------------

SQL INNER JOIN Example :

The "Persons" table:

P_Id LastName FirstName Address City
-----------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The "Orders" table:

O_Id OrderNo P_Id
-----------------
1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Now we want to list all the persons with any orders.

We use the following SELECT statement:

SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
INNER JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName

The result-set will look like this:

LastName FirstName OrderNo
--------------------------
Hansen Ola 22456
Hansen Ola 24562
Pettersen Kari 77895
Pettersen Kari 44678

SQL Joins

SQL joins are used to query data from two or more tables, based on a relationship between certain columns in these tables.
-------------------------------------------

SQL JOIN :

The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.

Tables in a database are often related to each other with keys.

A primary key is a column (or a combination of columns) with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.

Look at the "Persons" table:

P_Id LastName FirstName Address City
---------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Note that the "P_Id" column is the primary key in the "Persons" table. This means that no two rows can have the same P_Id. The P_Id distinguishes two persons even if they have the same name.

Next, we have the "Orders" table:

O_Id OrderNo P_Id
-------------------
1 77895 3
2 44678 3
3 22456 1
4 24562 1
5 34764 15

Note that the "O_Id" column is the primary key in the "Orders" table and that the "P_Id" column refers to the persons in the "Persons" table without using their names.

Notice that the relationship between the two tables above is the "P_Id" column.
-------------------

Different SQL JOINs :

Before we continue with examples, we will list the types of JOIN you can use, and the differences between them.

JOIN: Return rows when there is at least one match in both tables

LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table

RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table

FULL JOIN: Return rows when there is a match in one of the tables

SQL Alias

With SQL, an alias name can be given to a table or to a column.
----------------------------

SQL Alias :

You can give a table or a column another name by using an alias. This can be a good thing to do if you have very long or complex table names or column names.

An alias name could be anything, but usually it is short.

SQL Alias Syntax for Tables :

SELECT column_name(s)
FROM table_name
AS alias_name

SQL Alias Syntax for Columns :

SELECT column_name AS alias_name
FROM table_name
----------------------------------
Alias Example :

Assume we have a table called "Persons" and another table called "Product_Orders". We will give the table aliases of "p" an "po" respectively.

Now we want to list all the orders that "Ola Hansen" is responsible for.

We use the following SELECT statement:

SELECT po.OrderID, p.LastName, p.FirstName
FROM Persons AS p,
Product_Orders AS po
WHERE p.LastName='Hansen'
WHERE p.FirstName='Ola'

The same SELECT statement without aliases:

SELECT Product_Orders.OrderID, Persons.LastName, Persons.FirstName
FROM Persons,
Product_Orders
WHERE Persons.LastName='Hansen'
WHERE Persons.FirstName='Ola'

As you'll see from the two SELECT statements above; aliases can make queries easier to both write and to read.

SQL BETWEEN Operator

The BETWEEN operator is used in a WHERE clause to select a range of data between two values.
-------------------------------

The BETWEEN Operator :

The BETWEEN operator selects a range of data between two values. 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 "Persons" table:

P_Id LastName FirstName Address City
-------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the persons with a last name alphabetically between "Hansen" and "Pettersen" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName
BETWEEN 'Hansen' AND 'Pettersen'

The result-set will look like this:

P_Id LastName FirstName Address City
-----------------------------------
1 Hansen Ola Timoteivn 10 Sandnes

Note: The BETWEEN operator is treated differently in different databases.

In some databases, persons with the LastName of "Hansen" or "Pettersen" will not be listed, because the BETWEEN operator only selects fields that are between and excluding the test values).

In other databases, persons with the LastName of "Hansen" or "Pettersen" will be listed, because the BETWEEN operator selects fields that are between and including the test values).

And in other databases, persons with the LastName of "Hansen" will be listed, but "Pettersen" will not be listed (like the example above), because the BETWEEN operator 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.
-----------------------------------

Example 2 :

To display the persons outside the range in the previous example, use NOT BETWEEN:

SELECT * FROM Persons
WHERE LastName
NOT BETWEEN 'Hansen' AND 'Pettersen'

The result-set will look like this:

P_Id LastName FirstName Address City
-------------------------------------
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

The IN Operator

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 "Persons" table:

P_Id LastName FirstName Address City
------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Now we want to select the persons with a last name equal to "Hansen" or "Pettersen" from the table above.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName IN ('Hansen','Pettersen')

The result-set will look like this:

P_Id LastName FirstName Address City
------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

SQL Wildcards

SQL wildcards can be used when searching for data in a database.
---------------------------------------------------------
SQL Wildcards :

SQL wildcards can substitute for one or more characters when searching for data in a database.

SQL wildcards must be used with the SQL LIKE operator.

With SQL, the following wildcards can be used:

Wildcard Description :

% A substitute for zero or more characters
_ A substitute for exactly one character
[charlist] Any single character in charlist
[^charlist]
or

[!charlist]
Any single character not in charlist
-------------------------------------

SQL Wildcard Examples :
We have the following "Persons" table:

P_Id LastName FirstName Address City
------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger
------------------------------------
Using the % Wildcard
Now we want to select the persons living in a city that starts with "sa" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE City LIKE 'sa%'

The result-set will look like this:

P_Id LastName FirstName Address City
-------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes

Next, we want to select the persons living in a city that contains the pattern "nes" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE City LIKE '%nes%'

The result-set will look like this:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
-------------------------------------

Using the _ Wildcard
Now we want to select the persons with a first name that starts with any character, followed by "la" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE FirstName LIKE '_la'

The result-set will look like this:

P_Id LastName FirstName Address City
------------------------------------
1 Hansen Ola Timoteivn 10 Sandnes

Next, we want to select the persons with a last name that starts with "S", followed by any character, followed by "end", followed by any character, followed by "on" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName LIKE 'S_end_on'

The result-set will look like this:

P_Id LastName FirstName Address City
-------------------------------------
2 Svendson Tove Borgvn 23 Sandnes

-------------------------------------
Using the [charlist] Wildcard
Now we want to select the persons with a last name that starts with "b" or "s" or "p" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName LIKE '[bsp]%'

The result-set will look like this:

P_Id LastName FirstName Address City
------------------------------------
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

Next, we want to select the persons with a last name that do not start with "b" or "s" or "p" from the "Persons" table.

We use the following SELECT statement:

SELECT * FROM Persons
WHERE LastName LIKE '[!bsp]%'

The result-set will look like this:

P_Id LastName FirstName Address City
-----------------------------------
1 Hansen Ola Timoteivn 10 Sandnes