Joins

 

Let’s start with joins.

 

Like the name suggests, a join is used to join data from different tables based on a related column between the tables.

 

The syntax for joining two tables is:

 

SELECT [table_names.]columns_names_or_other_information

FROM

left_table

JOIN / INNER JOIN / LEFT JOIN / RIGHT JOIN

right_table

ON

left_table.column_name = right_table.column_name;

 

There are three main types of joins in mySQL: inner join, left join and right join. These are represented by the Venn diagrams below: To demonstrate the difference between them, let’s consider the following two tables:

 

Here, we have two tables called one and two.

 

You can see that column A in table one shares some common values with column C in table two. We can join the two tables using these two columns.

 

If we do an inner join, we write

 

SELECT A, C, one.B AS 'one B', two.B AS 'two B'

FROM

one

INNER...