SQL中INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN的使用

1. SQL JOIN

a brief introdction

The SQL JOIN clause is based on common fields between tables, and joining rows from multiple tables is to combine them.

classification

INNER JOIN: If there is at least one match in the table, the row is returned.

LEFT JOIN: Even if there is no match in the right table, all rows are returned from the left table.

RIGHT JOIN: Even if there is no match in the left table, all rows are returned from the right table.

FULL JOIN: As long as there is a match in one table, the row is returned.

2. INNER JOIN

database

Use the following database to explain:

Two tables, table_1, table_2.

As follows: table_1:

id	Name
1	Google
2	Taobao
3	Weibo
4	Facebook

table_2：

id	Address
1	United States
3	China
5	China
6	United States

grammar

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;
或者
SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;

Icon

From this figure, we can see that the data taken out by INNER JOIN is the data that the two tables intersect.

Statement example

select * from table_1 
inner join table_2
on table_1.id=table_2.id

The output of executing the above statement is as follows:

id	Name	Address
1	Google	United States
3	Weibo	China

3. LEFT JOIN

The LEFT JOIN keyword returns all rows from the left table (table_1), even if there is no match in the right table (table_2). If there is no match in the right table, the result is NULL.

grammar

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;

Icon

Statement example

select * from table_1 
left join table_2
on table_1.id=table_2.id

The output of executing the above statement is as follows:

id	Name	Address
1	Google	United States
2	Taobao	null
3	Weibo	China
4	Facebook	null

4. RIGHT JOIN

The RIGHT JOIN keyword will return all rows from the right table (table_2), even if there is no match in the left table (table_1). If there is no match in the left table, the result is NULL.

grammar

SELECT column_name(s)
FROM table_1
RIGHT JOIN table_2
ON table_1.column_name=table_2.column_name;

Icon

Statement example

select * from table_1 
right join table_2
on table_1.id=table_2.id

The output of executing the above statement is as follows:

id	Name	Address
1	Google	United States
3	Weibo	China
5	null	China
6	null	United States

5. FULL JOIN

FULL JOIN will return all rows from the left and right tables. If the data row of one table does not have a matching row in the other table, then the opposite data is replaced with NULL.

grammar

select column_name(s)
from table 1
FULL OUTER JOIN table 2
ON table 1.column_name=table 2.column_name

Icon

Statement example

select * from table_1 
full outer join table_2
on table_1.id=table_2.id

The output of executing the above SQL is as follows:

id	name	address
1	Google	United States
2	Taobao	null
3	Weibo	China
4	Facebook	null
5	null	China
6	null	United States

The picture comes from the rookie tutorial.