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 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
We use the following SELECT statement:
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM Persons
INNER JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName
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).
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
FROM Persons
LEFT JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName
SQL RIGHT JOIN Keyword
The RIGHT JOIN keyword Return all rows from the right table (table_name2), even if there are no matches in the left table (table_name1).
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM Persons
RIGHT JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName
SQL FULL JOIN Keyword
The FULL JOIN keyword return rows when there is a match in one of the tables.
SELECT Persons.LastName, Persons.FirstName, Orders.OrderNoFROM Persons
FULL JOIN Orders
ON Persons.P_Id=Orders.P_Id
ORDER BY Persons.LastName
The SQL UNION Operator
The UNION operator is used to combine the result-set of two or more SELECT statements.
Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.
Look at the following tables:
"Employees_Norway":
E_ID | E_Name |
---|---|
01 | Hansen, Ola |
02 | Svendson, Tove |
03 | Svendson, Stephen |
04 | Pettersen, Kari |
"Employees_USA":
E_ID | E_Name |
---|---|
01 | Turner, Sally |
02 | Kent, Clark |
03 | Svendson, Stephen |
04 | Scott, Stephen |
Now we want to list all the different employees in Norway and USA.
We use the following SELECT statement:
SELECT E_Name FROM Employees_Norway UNION SELECT E_Name FROM Employees_USA |
The result-set will look like this:
E_Name |
---|
Hansen, Ola |
Svendson, Tove |
Svendson, Stephen |
Pettersen, Kari |
Turner, Sally |
Kent, Clark |
Scott, Stephen |
SQL UNION ALL Example
Now we want to list all employees in Norway and USA:
SELECT E_Name FROM Employees_Norway UNION ALL SELECT E_Name FROM Employees_USA |
Result
E_Name |
---|
Hansen, Ola |
Svendson, Tove |
Svendson, Stephen |
Pettersen, Kari |
Turner, Sally |
Kent, Clark |
Svendson, Stephen |
Scott, Stephen |
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