A popular interpretation of several federated Mysql queries _mysql

Table A

Aid adate 
1 A1 
2 A2 
3 A3 

Table B

Bid bdate 
1 B1 
2 b2 
4 b4 

Two tables A, B connected, to remove the same ID field.

SELECT * from a INNER join B on a.aid = B.bid

This is the only matching data to be fetched.

At this point, the removal is:

1 A1 B1 
2 A2 B2 

So the left join means:

SELECT * from a LEFT join B on a.aid = B.bid 

First remove all the data from table A, then add the data that matches a and B.

At this point, the removal is:

1 A1 B1 
2 A2 b2 
3 A3 null characters

There is also the right join

Refers to the first to remove all the data in table B, and then add a and b matching data.

At this point, the removal is:

1 A1 B1 
2 A2 B2 
4 Null characters b4

Left JOIN or left OUTER join.

The result set of a left outer join consists of all the rows of the left table specified in the OUTER clause, not just the rows that the join columns match. If a row in the left table does not have a matching row in the right table, all picklist columns in the right table in the associated result set row are null values.

Instance

"Persons" table:

Address City

id_p	LastName	FirstName
1	Adams	John	Oxford Street	London
2	Bush	George	Fifth Avenue	New York
3	Carter	Thomas	Changan Street	Beijing

Orders table:

Id_o	OrderNo	id_p
1	77895	3
2	44678	3
3	22456	1
4	24562	1
5	34764	65

Left JOIN connection (left JOIN) instance

Now, we want to list all the people, as well as their order-if any.

You can use the following SELECT statement:

SELECT Persons.lastname, Persons.firstname, Orders.orderno from
Persons left
JOIN Orders on
persons.id_p= Orders.id_p ORDER BY
Persons.lastname

Result set:

LastName	FirstName	OrderNo
Adams	John	22456
Adams	John	24562
Carter	Thomas	77895
Carter	Thomas	44678
Bush	George

The left JOIN keyword returns all rows from the left-hand table (Persons), even if there are no matching rows in the right table (Orders).