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Description

The JOIN statement joins two tables ON a defined column. It is a regular JOIN used throughout SQL.

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Syntax

Here is the syntax: 
SELECT t1.column_name, t2.column_name, ...
FROM integration_name.table_name [AS] t1
JOIN integration_name.table_name [AS] t2
ON t1.column_name = t2.column_name;
On execution, we get: 
+-----------------+-----------------+
| t1.column_name  | t2.column_name  |
+-----------------+-----------------+
| t1.value        | t2.value        |
+-----------------+-----------------+
Where:
NameDescription
t1.column_nameName of the column from the first table.
t2.column_nameName of the column from the second table.
integration_name.table_nameName of the table used in the JOIN operation.
Nested JOINsMindsDB provides you with two categories of JOINs. One is the JOIN statement which combines the data table with the model table in order to fetch bulk predictions. Another is the regular JOIN used throughout SQL. Please note that only the latter one requires the ON clause.You can nest these types of JOINs as follows:
SELECT * FROM (
    SELECT *
    FROM project_name.model_table AS m
    JOIN integration_name.data_table AS d;
) AS t1
JOIN (
    SELECT *
    FROM project_name.model_table AS m
    JOIN integration_name.data_table AS d;
) AS t2
ON t1.column_name = t2.column_name;

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Example 1

Let’s use the following data to see how the different types of JOINs work. The pets table that stores pets: 
+------+-------+
|pet_id|name   |
+------+-------+
|1     |Moon   |
|2     |Ripley |
|3     |Bonkers|
|4     |Star   |
|5     |Luna   |
|6     |Lake   |
+------+-------+
And the owners table that stores pets’ owners: 
+--------+-------+------+
|owner_id|name   |pet_id|
+--------+-------+------+
|1       |Amy    |4     |
|2       |Bob    |1     |
|3       |Harry  |5     |
|4       |Julia  |2     |
|5       |Larry  |3     |
|6       |Henry  |0     |
+--------+-------+------+

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JOIN or INNER JOIN

The JOIN or INNER JOIN command joins the rows of the owners and pets tables wherever there is a match. For example, a pet named Lake does not have an owner, so it’ll be left out. 
SELECT *
FROM files.owners o
[INNER] JOIN files.pets p
ON o.pet_id = p.pet_id;
On execution, we get: 
+--------+-------+------+------+-------+
|owner_id|name   |pet_id|pet_id|name   |
+--------+-------+------+------+-------+
|1       |Amy    |4     |4     |Star   |
|2       |Bob    |1     |1     |Moon   |
|3       |Harry  |5     |5     |Luna   |
|4       |Julia  |2     |2     |Ripley |
|5       |Larry  |3     |3     |Bonkers|
+--------+-------+------+------+-------+
As in standard SQL, you can use the WHERE clause to filter the output data. 
SELECT *
FROM files.owners o
[INNER] JOIN files.pets p
ON o.pet_id = p.pet_id
WHERE o.name = 'Amy'
OR o.name = 'Bob';
On execution, we get: 
+--------+-------+------+------+-------+
|owner_id|name   |pet_id|pet_id|name   |
+--------+-------+------+------+-------+
|1       |Amy    |4     |4     |Star   |
|2       |Bob    |1     |1     |Moon   |
+--------+-------+------+------+-------+

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LEFT JOIN

The LEFT JOIN command joins the rows of two tables such that all rows from the left table, even the ones with no match, show up. Here, the left table is the owners table. 
SELECT *
FROM files.owners o
LEFT JOIN files.pets p
ON o.pet_id = p.pet_id;
On execution, we get: 
+--------+-------+------+------+-------+
|owner_id|name   |pet_id|pet_id|name   |
+--------+-------+------+------+-------+
|1       |Amy    |4     |4     |Star   |
|2       |Bob    |1     |1     |Moon   |
|3       |Harry  |5     |5     |Luna   |
|4       |Julia  |2     |2     |Ripley |
|5       |Larry  |3     |3     |Bonkers|
|6       |Henry  |0     |[NULL]|[NULL] |
+--------+-------+------+------+-------+

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RIGHT JOIN

The RIGHT JOIN command joins the rows of two tables such that all rows from the right table, even the ones with no match, show up. Here, the right table is the pets table. 
SELECT *
FROM files.owners o
RIGHT JOIN files.pets p
ON o.pet_id = p.pet_id;
On execution, we get: 
+--------+-------+------+------+-------+
|owner_id|name   |pet_id|pet_id|name   |
+--------+-------+------+------+-------+
|2       |Bob    |1     |1     |Moon   |
|4       |Julia  |2     |2     |Ripley |
|5       |Larry  |3     |3     |Bonkers|
|1       |Amy    |4     |4     |Star   |
|3       |Harry  |5     |5     |Luna   |
|[NULL]  |[NULL] |[NULL]|6     |Lake   |
+--------+-------+------+------+-------+

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FULL JOIN or FULL OUTER JOIN

The FULL [OUTER] JOIN command joins the rows of two tables such that all rows from both tables, even the ones with no match, show up. 
SELECT *
FROM files.owners o
FULL [OUTER] JOIN files.pets p
ON o.pet_id = p.pet_id;
On execution, we get: 
+--------+------+------+------+-------+---------+
|owner_id|name  |pet_id|pet_id|name   |animal_id|
+--------+------+------+------+-------+---------+
|1       |Amy   |4     |4     |Star   |2        |
|2       |Bob   |1     |1     |Moon   |1        |
|3       |Harry |5     |5     |Luna   |2        |
|4       |Julia |2     |2     |Ripley |1        |
|5       |Larry |3     |3     |Bonkers|3        |
|6       |Henry |0     |[NULL]|[NULL] |[NULL]   |
|[NULL]  |[NULL]|[NULL]|6     |Lake   |4        |
+--------+------+------+------+-------+---------+

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Example 2

More than two tables can be joined subsequently. Let’s use another table called animals: 
+---------+-------+
|animal_id|name   |
+---------+-------+
|1        |Dog    |
|2        |Cat    |
|3        |Hamster|
|4        |Fish   |
+---------+-------+
Now we can join all three tables. 
SELECT *
FROM files.owners o
RIGHT JOIN files.pets p ON o.pet_id = p.pet_id
JOIN files.animals a ON p.animal_id = a.animal_id;
On execution, we get: 
+--------+-------+------+------+-------+---------+---------+-------+
|owner_id|name   |pet_id|pet_id|name   |animal_id|animal_id|name   |
+--------+-------+------+------+-------+---------+---------+-------+
|2       |Bob    |1     |1     |Moon   |1        |1        |Dog    |
|4       |Julia  |2     |2     |Ripley |1        |1        |Dog    |
|5       |Larry  |3     |3     |Bonkers|3        |3        |Hamster|
|1       |Amy    |4     |4     |Star   |2        |2        |Cat    |
|3       |Harry  |5     |5     |Luna   |2        |2        |Cat    |
|[NULL]  |[NULL] |[NULL]|6     |Lake   |4        |4        |Fish   |
+--------+-------+------+------+-------+---------+---------+-------+
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