SQL Joins

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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.

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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

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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.

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