As we have matching values of person_id in both the left and right table (both tables have person_id=1 and person_id=2), we retrieve the first_name and last_name for each of these two people.
For person_id=3 this only exists in the left table and there is no match in the right table, hence person_id=3 is not included in the table that results from the INNER JOIN.

LEFT JOIN

MySQL's LEFT JOIN clause returns all rows from the left table and records that match from the right table. NULL will be used for records that have no matching record in the right table.

Consider again the very simplistic example where we have the following two tables:

Left table:

person_id	first_name
1	Bob
2	Mary
3	Craig

Right table:

person_id	last_name
1	Builder
2	Poppins

Table that results from joining the two tables using LEFT JOIN on the common column person_id:

person_id	first_name	last_name
1	Bob	Builder
2	Mary	Poppins
3	Craig	NULL

Here we retrieve all rows from the left table (i.e. person_id=1, person_id=2, person_id=3) regardless of whether they have a matching row in the right table.
Next we retrieve the corresponding last_name for each person_id from the right table.
As person_id=3 does not have a corresponding last_name in the right table, the value is populated as NULL.

RIGHT JOIN

MySQL's RIGHT JOIN clause returns all rows from the right table and matching records from the left table. NULL will be used for records that have no matching record in the left table.

Consider again the very simplistic example where we have the following two tables:

Left table:

person_id	first_name
1	Bob
2	Mary

Right table:

person_id	last_name
1	Builder
2	Poppins
3	Grey

Table that results from joining the two tables using RIGHT JOIN on the common column person_id:

person_id	first_name	last_name
1	Bob	Builder
2	Mary	Poppins
3	NULL	Grey

Here we retrieve all rows from the right table (i.e. person_id=1, person_id=2, person_id=3) regardless of whether they have a matching row in the left table.
Next we retrieve the corresponding first_name for each person_id from the left table.
As person_id=3 does not have a corresponding first_name in the left table, the value is populated as NULL.