# JOINs

This page explains the syntax used to write join. In order to get a more in deep knowledge of how joins work it is recommended to read [Optimizing joins](https://docs.pinot.apache.org/release-1.3.0/for-users/user-guide-query/multi-stage-query/optimizing-joins) and also [this blog](https://startree.ai/resources/query-time-joins-in-apache-pinot-1-0) from Star Tree.

{% hint style="info" %}
**Important:** To query using JOINs, you must [use Pinot's multi-stage query engine (v2).](https://docs.pinot.apache.org/release-1.3.0/reference/multi-stage-engine)
{% endhint %}

## Inner join

The inner join selects rows that have matching values in both tables.

### **Syntax:**

{% code overflow="wrap" %}

```sql
SELECT myTable.column1,myTable.column2,myOtherTable.column1,....
FROM mytable INNER JOIN table2
ON table1.matching_column = myOtherTable.matching_column;
```

{% endcode %}

### Example of inner join

Joins a table containing user transactions with a table containing promotions shown to the users, to show the spending for every userID.

{% code overflow="wrap" %}

```sql
SELECT 
  p.userID, t.spending_val

FROM promotion AS p JOIN transaction AS t 
  ON p.userID = t.userID

WHERE
  p.promotion_val > 10
  AND t.transaction_type IN ('CASH', 'CREDIT')  
  AND t.transaction_epoch >= p.promotion_start_epoch
  AND t.transaction_epoch < p.promotion_end_epoch  
```

{% endcode %}

## Left join

A left join returns all values from the left relation and the matched values from the right table, or appends NULL if there is no match. Also referred to as a left outer join.

### **Syntax:**

{% code overflow="wrap" %}

```sql
SELECT myTable.column1,table1.column2,myOtherTable.column1,....
FROM myTable LEFT JOIN myOtherTable
ON myTable.matching_column = myOtherTable.matching_column;
```

{% endcode %}

## R**ight join**

A right join returns all values from the right relation and the matched values from the left relation, or appends NULL if there is no match. It is also referred to as a right outer join.

**Syntax:**

{% code overflow="wrap" %}

```sql
SELECT table1.column1,table1.column2,table2.column1,....
FROM table1 
RIGHT JOIN table2
ON table1.matching_column = table2.matching_column;
```

{% endcode %}

## F**ull join**

A full join returns all values from both relations, appending NULL values on the side that does not have a match. It is also referred to as a full outer join.

### **Syntax:**

{% code overflow="wrap" %}

```sql
SELECT table1.column1,table1.column2,table2.column1,....
FROM table1 
FULL JOIN table2
ON table1.matching_column = table2.matching_column;
```

{% endcode %}

## Cross join

A cross join returns the Cartesian product of two relations. If no WHERE clause is used along with CROSS JOIN, this produces a result set that is the number of rows in the first table multiplied by the number of rows in the second table. If a WHERE clause is included with CROSS JOIN, it functions like an [INNER JOIN](#inner-join).

### **Syntax:**

```sql
SELECT * 
FROM table1 
CROSS JOIN table2;
```

## Semi/A**nti join**

Semi/anti-join returns rows from the first table where no matches are found in the second table. Returns one copy of each row in the first table for which no match is found.

### **Syntax:**

```sql
SELECT  myTable.column1, myOtherTable.column1
 FROM  myOtherTable
 WHERE  NOT EXISTS [ join_criteria ]
```

Some subqueries, like the following are also implemented as a semi-join under the hood:

```sql
SELECT table1.strCol
 FROM  table1
 WHERE table1.intCol IN (select table2.anotherIntCol from table2 where ...)
```

## Equi join

An equi join uses an equality operator to match a single or multiple column values of the relative tables.&#x20;

### **Syntax:**

```sql
SELECT *
FROM table1 
JOIN table2
[ON (join_condition)]

OR

SELECT column_list 
FROM table1, table2....
WHERE table1.column_name =
table2.column_name; 
```