untung99.homes: SQL JOIN With Examples
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The SQL JOIN
joins two tables based on a common column and selects records that have matching values in these columns.
-- join the Customers and Orders tables
-- based on the common values of their customer_id columns
SELECT Customers.customer_id, Customers.first_name, Orders.item
FROM Customers
JOIN Orders
ON Customers.customer_id = Orders.customer_id;
Here, the SQL command joins the Customers
and Orders
tables based on the common values in the customer_id
columns of both tables.
The result set will consist of
customer_id
andfirst_name
columns from theCustomers
tableitem
column from theOrders
table
SQL JOIN Syntax
The syntax of the SQL JOIN
statement is:
SELECT columns_from_both_tables
FROM table1
JOIN table2
ON table1.column1 = table2.column2
Here,
- table1 and table2 are the two tables that are to be joined
- column1 is the column in table1 that is related to column2 in table2
Note: There are 4 types of JOINs in SQL. But INNER JOIN
and JOIN
refer to the same thing.
Example 1: SQL JOIN
-- join Customers and Orders tables based on
-- customer_id of Customers and customer column of Orders
SELECT Customers.customer_id, Customers.first_name, Orders.amount
FROM Customers
JOIN Orders
ON Customers.customer_id = Orders.customer;
Here’s how this code works:
Here, the SQL command selects the customer_id
and first_name
columns (from the Customers
table) and the amount
column (from the Orders
table).
The result set will contain those rows where there is a match between customer_id
(of the Customers
table) and customer
(of the Orders
table).
Types of SQL JOINs
As we mentioned, the JOIN
command we performed in this article is INNER JOIN
.
In SQL, we have four main types of joins:
SQL JOIN and Aliases
We can use AS aliases with table names to make our query short and clean. For example,
-- use alias C for Customers table
-- use alias O for Orders table
SELECT C.customer_id, C.first_name, O.amount
FROM Customers AS C
JOIN Orders AS O
ON C.customer_id = O.customer;
Here, the SQL command joins the Customers
and Orders
tables while assigning the aliases C and O to them, respectively.
Also, we can change the column names temporarily using AS
aliases. For example,
-- use alias C for Customers table
-- use alias O for Orders table
SELECT C.customer_id AS cid, C.first_name AS name, O.amount
FROM Customers AS C
JOIN Orders AS O
ON C.customer_id = O.customer;
Apart from giving aliases to the tables, the SQL command above also assigns aliases to the columns of the Customers
table:
customer_id
column has the aliascid
first_name
column has the aliasname