JOIN
A join allows you to combine columns from two or more tables into a single result set. Databend supports the following join types:
- Inner Join
- Natural Join
- Cross Join
- Left Join
- Right Join
Inner Join
An inner join returns the rows that meet the join conditions in the result set.
Syntax
SELECT select_list
FROM table_a
[INNER] JOIN table_b
ON join_condition_1
[[INNER] JOIN table_c
ON join_condition_2]...
The INNER keyword is optional.
When you join two tables on a common column with the equal operator, you can use the USING keyword to simplify the syntax.
SELECT select_list
FROM table_a
JOIN table_b
USING join_column_1
[JOIN table_c
USING join_column_2]...
Examples
Imagine we have the following tables:
Table "vip_info": This table stores the VIP client information.
| Client_ID | Region |
|---|---|
| 101 | Toronto |
| 102 | Quebec |
| 103 | Vancouver |
Table "purchase_records": This table lists the purchase records for all the clients.
| Client_ID | Item | QTY |
|---|---|---|
| 100 | Croissant | 2,000 |
| 102 | Donut | 3,000 |
| 103 | Coffee | 6,000 |
| 106 | Soda | 4,000 |
The following command returns the purchase records of the VIP clients:
select purchase_records.Client_ID, purchase_records.Item, purchase_records.QTY from vip_info inner join purchase_records on vip_info.Client_ID = purchase_records.Client_ID;
Output:
|102|Donut|3000
|103|Coffee|6000
Natural Join
A natural join joins two tables based on all columns in the two tables that have the same name.
Syntax
SELECT select_list
FROM table_a
NATURAL JOIN table_b
[NATURAL JOIN table_c]...
Examples
Imagine we have the following tables:
Table "vip_info": This table stores the VIP client information.
| Client_ID | Region |
|---|---|
| 101 | Toronto |
| 102 | Quebec |
| 103 | Vancouver |
Table "purchase_records": This table lists the purchase records for all the clients.
| Client_ID | Item | QTY |
|---|---|---|
| 100 | Croissant | 2,000 |
| 102 | Donut | 3,000 |
| 103 | Coffee | 6,000 |
| 106 | Soda | 4,000 |
The following command returns the purchase records of the VIP clients:
select purchase_records.Client_ID, purchase_records.Item, purchase_records.QTY from vip_info natural join purchase_records;
Output:
|102|Donut|3,000
|103|Coffee|6,000
Cross Join
A cross join returns a result set that includes each row from the first table joined with each row from the second table.
Syntax
SELECT select_list
FROM table_a
CROSS JOIN table_b
Examples
Imagine we have the following tables:
Table "vip_info": This table stores the VIP client information.
| Client_ID | Region |
|---|---|
| 101 | Toronto |
| 102 | Quebec |
| 103 | Vancouver |
Table "gift": This table lists the gift options for the VIP clients.
| Gift |
|---|
| Croissant |
| Donut |
| Coffee |
| Soda |
The following command returns a result set that assigns each gift option to each VIP client:
select * from vip_info cross join gift;
Output:
101|Toronto|Croissant
101|Toronto|Donut
101|Toronto|Coffee
101|Toronto|Soda
102|Quebec|Croissant
102|Quebec|Donut
102|Quebec|Coffee
102|Quebec|Soda
103|Vancouver|Croissant
103|Vancouver|Donut
103|Vancouver|Coffee
103|Vancouver|Soda
Left Join
A left join returns all records from the left table, and the matching records from the right table. The result is NULL records from the right side, if there is no match.
Syntax
SELECT select_list
FROM table_a
LEFT [OUTER] JOIN table_b
ON join_condition_1
The OUTER keyword is optional.
Examples
Imagine we have the following tables:
Table "vip_info": This table stores the VIP client information.
| Client_ID | Region |
|---|---|
| 101 | Toronto |
| 102 | Quebec |
| 103 | Vancouver |
Table "purchase_records": This table lists the purchase records for all the clients.
| Client_ID | Item | QTY |
|---|---|---|
| 100 | Croissant | 2,000 |
| 102 | Donut | 3,000 |
| 103 | Coffee | 6,000 |
| 106 | Soda | 4,000 |
The following command returns the purchase records of all VIP clients, the purchase records will be NULL if the VIP client has no purchases:
select vip_info.Client_ID, purchase_records.Item, purchase_records.QTY from vip_info left join purchase_records on vip_info.Client_ID = purchase_records.Client_ID;
Output:
|101|NULL|NULL
|102|Donut|3000
|103|Coffee|6000
Right Join
A right join returns all records from the right table, and the matching records from the left table. The result is NULL records from the left side, if there is no match.
Syntax
SELECT select_list
FROM table_a
RIGHT [OUTER] JOIN table_b
ON join_condition_1
The OUTER keyword is optional.
Examples
Imagine we have the following tables:
Table "vip_info": This table stores the VIP client information.
| Client_ID | Region |
|---|---|
| 101 | Toronto |
| 102 | Quebec |
| 103 | Vancouver |
Table "purchase_records": This table lists the purchase records for all the clients.
| Client_ID | Item | QTY |
|---|---|---|
| 100 | Croissant | 2,000 |
| 102 | Donut | 3,000 |
| 103 | Coffee | 6,000 |
| 106 | Soda | 4,000 |
The following command returns all vip_info of all purchase_records, the vip_info will be NULL if purchase_record does not have the corresponding vip_info.
select vip_info.Client_ID, vip_info.Region from vip_info right join purchase_records on vip_info.Client_ID = purchase_records.Client_ID;
Output:
NULL|NULL
102|Quebec
103|Vancouver
NULL|NULL