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PostgreSQL - FULL OUTER JOIN

In PostgreSQL, a FULL OUTER JOIN returns all the records when there is a match in either the left (first) table or the right (second) table records. If there's no match, the result is NULL for every column of the table that lacks a matching row.

Syntax:

SELECT columns
FROM table1
FULL OUTER JOIN table2
ON table1.column = table2.column;

Example:

Let's consider two tables:

employees

| id | name      |
|----|-----------|
| 1  | John      |
| 2  | Lucy      |
| 3  | Mike      |

orders

| order_id | employee_id | product  |
|----------|-------------|----------|
| 101      | 1           | Laptop   |
| 102      | 3           | Monitor  |
| 103      | 5           | Keyboard |

Now, if you want to list all employees and their orders, even if they haven't made any orders or there are orders with non-existent employee IDs, you'd use a FULL OUTER JOIN:

SELECT e.name, o.product
FROM employees e
FULL OUTER JOIN orders o ON e.id = o.employee_id;

The result would be:

| name | product  |
|------|----------|
| John | Laptop   |
| Lucy | NULL     |
| Mike | Monitor  |
| NULL | Keyboard |

Here's what happens:

John has made an order, so his name and product ("Laptop") are listed.
Lucy hasn't made any orders, so her name appears with a NULL in the product column.
Mike has made an order, so his name and product ("Monitor") are listed.
There's an order (Keyboard) with an employee_id (5) that doesn't exist in the employees table, so NULL appears in the name column and the product is shown.

The FULL OUTER JOIN ensures that we don't miss data from either table, filling in gaps with NULL where necessary.

How to use FULL OUTER JOIN in PostgreSQL:
- Description: Use FULL OUTER JOIN to combine rows from both tables, including unmatched rows from both tables.
- Code:
```
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column;
```
PostgreSQL FULL OUTER JOIN example:
- Description: An example of using FULL OUTER JOIN to retrieve all rows from both tables.
- Code:
```
SELECT *
FROM employees
FULL OUTER JOIN departments ON employees.department_id = departments.department_id;
```

Comparing INNER JOIN and FULL OUTER JOIN in PostgreSQL:

Description: Compare the results of INNER JOIN and FULL OUTER JOIN.

Code:

-- INNER JOIN
SELECT *
FROM table1
INNER JOIN table2 ON table1.column = table2.column;

-- FULL OUTER JOIN
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column;

Multiple tables FULL OUTER JOIN in PostgreSQL:

Description: Extend FULL OUTER JOIN to include multiple tables.

Code:

SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column
FULL OUTER JOIN table3 ON table1.column = table3.column;

Handling NULL values with FULL OUTER JOIN in PostgreSQL:
- Description: Address NULL values resulting from unmatched rows in FULL OUTER JOIN.
- Code:
```
SELECT table1.column, table2.column
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column;
```

FULL OUTER JOIN vs. LEFT JOIN vs. RIGHT JOIN in PostgreSQL:

Description: Compare FULL OUTER JOIN with LEFT JOIN and RIGHT JOIN.

Code:

-- LEFT JOIN
SELECT *
FROM table1
LEFT JOIN table2 ON table1.column = table2.column;

-- RIGHT JOIN
SELECT *
FROM table1
RIGHT JOIN table2 ON table1.column = table2.column;

-- FULL OUTER JOIN
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column;

Using WHERE clause with FULL OUTER JOIN in PostgreSQL:
- Description: Apply conditions in the WHERE clause to filter results from FULL OUTER JOIN.
- Code:
```
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column
WHERE table1.condition_column = 'value';
```
Conditional FULL OUTER JOIN in PostgreSQL:
- Description: Use conditional statements within FULL OUTER JOIN to control the joining condition.
- Code:
```
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column AND table2.condition_column = 'value';
```

Combining FULL OUTER JOIN with GROUP BY in PostgreSQL:

Description: Group results from FULL OUTER JOIN using GROUP BY.

Code:

SELECT column1, COUNT(column2)
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column
GROUP BY column1;

FULL OUTER JOIN with subqueries in PostgreSQL:

Description: Use subqueries in conjunction with FULL OUTER JOIN for more complex queries.

Code:

SELECT *
FROM (
    SELECT column1 FROM table1
    UNION
    SELECT column1 FROM table2
) AS subquery
FULL OUTER JOIN table3 ON subquery.column1 = table3.column1;

Applying ORDER BY with FULL OUTER JOIN results in PostgreSQL:
- Description: Order the results of FULL OUTER JOIN using the ORDER BY clause.
- Code:
```
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column
ORDER BY table1.column;
```
Handling duplicates in FULL OUTER JOIN in PostgreSQL:
- Description: Handle duplicate rows that may result from FULL OUTER JOIN.
- Code:
```
SELECT DISTINCT column1, column2
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column;
```

Nested FULL OUTER JOIN in PostgreSQL:

Description: Nest FULL OUTER JOIN within other join types for more complex queries.

Code:

SELECT *
FROM table1
FULL OUTER JOIN (table2 INNER JOIN table3 ON table2.column = table3.column) ON table1.column = table2.column;

Using aliases with FULL OUTER JOIN in PostgreSQL:
- Description: Employ aliases to simplify and clarify column references in FULL OUTER JOIN.
- Code:
```
SELECT t1.column AS t1_column, t2.column AS t2_column
FROM table1 t1
FULL OUTER JOIN table2 t2 ON t1.column = t2.column;
```

Optimizing queries with FULL OUTER JOIN in PostgreSQL:

Description: Optimize query performance by indexing columns used in the FULL OUTER JOIN condition.

Code:

CREATE INDEX idx_table1_column ON table1(column);
CREATE INDEX idx_table2_column ON table2(column);

SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column = table2.column;

Common mistakes with FULL OUTER JOIN in PostgreSQL:
- Description: Avoid common pitfalls, such as missing conditions or incorrect column references.
- Code:
```
-- Incorrect column reference
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.column1 = table2.column2;
```