MySQL Innerjoin left joins right join parsing

After graduating for more than half a year, have not learned to join before the joins have been to take the data from a table and then take the data from the other table, and then write a function loop format data.

Let's write about what you learned first.

Reprinted from W3school

Join and Key

Sometimes in order to get the complete result, we need to get the results from two or more tables. We need to execute the join.

Tables in the database can be linked by keys. The primary key (Primary key) is a column, and the value of each row in the column is unique. In the table, the value of each primary key is unique. The purpose of this is to cross-bind the data between tables without repeating all the data in each table.

Please see the "Persons" table:

City

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

Note that the "id_p" column is the primary key in the Persons table. This means that no two lines can have the same id_p. Even if the names of two people are exactly the same, id_p can distinguish them from each other.

Next, look at the Orders table:

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

Note that the "id_o" column is the primary key in the Orders table, and the "id_p" column in the Orders table is used to refer to people in the "Persons" table without using their exact names.

Please note that the "id_p" column links the above two tables.

Referencing two tables

We can get the data from two tables by referencing two tables:

Who ordered the products and what products they ordered?

SELECT Persons.lastname, Persons.firstname, Orders.orderno  from Persons, Orders WHERE =

Result set:

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

SELECT column_name (s)  from table_name1  Left JOIN   on table_name1.column_name=table_name2.column_name

SQL join-Using Join

In addition to the above method, we can also use keyword JOIN to get data from two tables.

If we want to list everyone's orders, you can use the following SELECT statement:

SELECT Persons.lastname, Persons.firstname, Orders.ordernofrom Persons on INNER JOIN Orders persons.id_p = Orders.id_porder by Persons.lastname

Result set:

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

SQL left JOIN keyword

The left JOIN keyword returns all rows from the table (TABLE_NAME1), even if there are no matching rows in the right table (table_name2).

Left JOIN keyword syntax

Note: In some databases, the left join is called the left OUTER join.

The original table (used in the example):

"Persons" table:

City

id_p	LastName	FirstName	Address
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 orders-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_pORDER 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 table (Persons), even if there are no matching rows in the right table (Orders)

SQL Right JOIN keyword

The right JOIN keyword returns all rows from the table (table_name2), even if there are no matching rows in the left table (table_name1).

Right JOIN keyword syntax

SELECT column_name (s)  from table_name1  Right JOIN   on table_name1.column_name=table_name2.column_name

Note: In some databases, the right join is called a OUTER join.

The original table (used in the example):

"Persons" table:

City

id_p	LastName	FirstName	Address
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

Right Join instance

Now, we want to list all the orders and the people who ordered them-if any.

You can use the following SELECT statement:

SELECT Persons.lastname, Persons.firstname, Orders.orderno  from Persons  Right JOIN Orders  on Persons.id_p=orders.id_pORDER by Persons.lastname

Result set:

LastName	FirstName	OrderNo
Adams	John	22456
Adams	John	24562
Carter	Thomas	77895
Carter	Thomas	44678
		34764

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