In a database such as MySQL, data is divided into a series of tables (the "why" is beyond what I'm writing today) which are then connected together in SELECT commands to generate the output required. I find when I'm running MySQL training, people often get confused between all the join flavours. Let me give you an example to see how it works.

If this isn't quite the question you're looking to have answered, we've got a MySQL IAQ (Infrequently Answered Questions that may help you. And if you want to learn how to make use of this in PHP, see here

First, some sample data:
Mr Brown, Person number 1, has a phone number 01225 708225
Miss Smith, Person number 2, has a phone number 01225 899360
Mr Pullen, Person number 3, has a phone number 01380 724040
and also:
Person number 1 is selling property number 1 - Old House Farm
Person number 3 is selling property number 2 - The Willows
Person number 3 is (also) selling property number 3 - Tall Trees
Person number 3 is (also) selling property number 4 - The Melksham Florist
Person number 4 is selling property number 5 - Dun Roamin.

mysql> select * from demo_people;

+------------+--------------+------+

| name       | phone        | pid  |

+------------+--------------+------+

| Mr Brown   | 01225 708225 |    1 |

| Miss Smith | 01225 899360 |    2 |

| Mr Pullen  | 01380 724040 |    3 |

+------------+--------------+------+

3 rows in set (0.00 sec)



mysql> select * from demo_property;

+------+------+----------------------+

| pid  | spid | selling              |

+------+------+----------------------+

|    1 |    1 | Old House Farm       |

|    3 |    2 | The Willows          |

|    3 |    3 | Tall Trees           |

|    3 |    4 | The Melksham Florist |

|    4 |    5 | Dun Roamin           |

+------+------+----------------------+

5 rows in set (0.00 sec)



mysql>

If I do a regular JOIN (with none of the keywords INNER, OUTER, LEFT or RIGHT), then I get all records that match in the appropriate way in the two tables, and records in both incoming tables that do not match are not reported:

mysql> select name, phone, selling 

from demo_people join demo_property 

on demo_people.pid = demo_property.pid;

+-----------+--------------+----------------------+

| name      | phone        | selling              |

+-----------+--------------+----------------------+

| Mr Brown  | 01225 708225 | Old House Farm       |

| Mr Pullen | 01380 724040 | The Willows          |

| Mr Pullen | 01380 724040 | Tall Trees           |

| Mr Pullen | 01380 724040 | The Melksham Florist |

+-----------+--------------+----------------------+

4 rows in set (0.01 sec)



mysql>

If I do a LEFT JOIN, I get all records that match in the same way and IN ADDITION I get an extra record for each unmatched record in the left table of the join - thus ensuring (in my example) that every PERSON gets a mention:

mysql> select name, phone, selling 

from demo_people left join demo_property 

on demo_people.pid = demo_property.pid; 

+------------+--------------+----------------------+

| name       | phone        | selling              |

+------------+--------------+----------------------+

| Mr Brown   | 01225 708225 | Old House Farm       |

| Miss Smith | 01225 899360 | NULL                 |

| Mr Pullen  | 01380 724040 | The Willows          |

| Mr Pullen  | 01380 724040 | Tall Trees           |

| Mr Pullen  | 01380 724040 | The Melksham Florist |

+------------+--------------+----------------------+

5 rows in set (0.00 sec)



mysql>

If I do a RIGHT JOIN, I get all the records that match and IN ADDITION I get an extra record for each unmatched record in the right table of the join - in my example, that means that each property gets a mention even if we don't have seller details:

mysql> select name, phone, selling 

from demo_people right join demo_property 

on demo_people.pid = demo_property.pid;

+-----------+--------------+----------------------+

| name      | phone        | selling              |

+-----------+--------------+----------------------+

| Mr Brown  | 01225 708225 | Old House Farm       |

| Mr Pullen | 01380 724040 | The Willows          |

| Mr Pullen | 01380 724040 | Tall Trees           |

| Mr Pullen | 01380 724040 | The Melksham Florist |

| NULL      | NULL         | Dun Roamin           |

+-----------+--------------+----------------------+

5 rows in set (0.00 sec)



mysql>

An INNER JOIN does a full join, just like the first example, and the word OUTER may be added after the word LEFT or RIGHT in the last two examples - it's provided for ODBC compatibility and doesn't add an extra capabilities.

inner join,full outer join,left join,right jion
内部连接 inner join 两表都满足的组合
full outer 全连 两表相同的组合在一起，A表有，B表没有的数据（显示为null）,同样B表有
A表没有的显示为(null)
A表 left join  B表 左连,以A表为基础，A表的全部数据，B表有的组合。没有的为null
A表 right join B表 右连,以B表为基础，B表的全部数据，A表的有的组合。没有的为null