SQL LECTURE 8

Lecture 8

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

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Alias Example

Assume we have a table called "Persons" and another table called "Product_Orders". We will give the table aliases of "p" and "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' AND 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' AND Persons.FirstName='Ola'

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

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

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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.

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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 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.

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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

The INNER JOIN keyword return rows when there is at least one match in both tables. If there are rows in "Persons" that do not have matches in "Orders", those rows will NOT be listed.