# UNION and UNION ALL

The `UNION` clause combines results from multiple `SELECT` statements. It automatically eliminates duplicate rows, delivering a distinct dataset crucial for consolidated reporting or unique data identification across different query outputs.

For effective use, each `SELECT ` statement within a `UNION` operation **must**:

*   Select the identical count of columns.
*   Include the same number of column expressions.
*   Maintain the same data type order across corresponding columns.

Column lengths do not need to match .

## The UNION Clause

Consider the following structures:

The `customers` table:

| id | name     | age | address   | salary   |
|----|----------|-----|-----------|----------|
|   1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|   3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00  |
|  7 | Muffy    |  24 | Indore    | 10000.00 |

The `orders` table:

| oid | date                | customer_id | amount |
|-----|---------------------|-------------|--------|
| 102 | 2009-10-08 00:00:00 |           3 |   3000 |
| 100  | 2009-10-08 00:00:00 |           3 |   1500 |
| 101 | 2009-11-2 0 00:00:00 |           2 |   1560 |
| 103 | 2008-05-20 00:00:00 |            4 |   2060 |

Combining results with `UNION`:

```sql
SELECT id, name, amount, date
FROM customers
LEFT JOIN orders
ON customers.id = orders.customer_id
UNION
SELECT id, name, amount, date
FROM customers
RIGHT JOIN orders
ON customers.id = orders.customer_id;
```

Resulting data set:

| id   | name     | amount | date                 |
|------|----------|--------|---------------------|
|    1 | Ramesh   |   NULL | NULL                |
|    2 | Khilan   |   1560 | 2009- 11-20 00:00:00 |
|    3 | kaushik  |   3000 | 2009-10-08 00:00 :00 |
|    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
|    4 | Chaitali  |   2060 | 2008-05-20 00:00:00 |
|    5 | Hardik   |   NULL | NULL                |
|    6 |  Komal    |   NULL | NULL                |
|    7 | Muffy    |   NULL | NULL                |

## The UNION ALL Clause

The `UNION ALL` operator concatenates the result sets of two or more `SELECT ` statements. It explicitly includes all rows, including duplicates, which is critical when the frequency of specific data points is important.

The requirements for column count, expressions, and data types mirroring across `SELECT` statements remain identical to `UNION`.

Using  the same `customers` and `orders` tables:

Combining results with `UNION ALL`:

```sql
SELECT id, name, amount, date
FROM customers
LEFT JOIN orders
ON customers.id = orders.customer _id
UNION ALL
SELECT id, name, amount, date
FROM customers
RIGHT JOIN orders
ON customers.id = orders.customer_id;
```

Resulting data set:

| id    | name     | amount | date                |
|------|----------|--------|---------------------|
|    1 | Ramesh   |   NULL | NULL                |
|    2 | Khilan   |   1560 |  2009-11-20 00:00:00 |
|    3 | kaushik  |   3000 | 2009-10-08  00:00:00 |
|    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
|    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
|    5 | Hardik   |   NULL | NULL                 |
|    6 | Komal    |   NULL | NULL                |
|    7 | Muffy   |   NULL | NULL                |
|    3 | kaushik  |   3000 |  2009-10-08 00:00:00 |
|    3 | kaushik  |   1500 | 2009-10-08  00:00:00 |
|    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
|     4 | Chaitali |   2060 | 2008-05-20 00:00:00 |