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103 lines
3.5 KiB
Markdown
103 lines
3.5 KiB
Markdown
# UNION and UNION ALL
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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.
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For effective use, each `SELECT ` statement within a `UNION` operation **must**:
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* Select the identical count of columns.
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* Include the same number of column expressions.
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* Maintain the same data type order across corresponding columns.
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Column lengths do not need to match .
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## The UNION Clause
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Consider the following structures:
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The `customers` table:
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| id | name | age | address | salary |
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|----|----------|-----|-----------|----------|
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| 1 | Ramesh | 32 | Ahmedabad | 2000.00 |
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| 2 | Khilan | 25 | Delhi | 1500.00 |
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| 3 | kaushik | 23 | Kota | 2000.00 |
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| 4 | Chaitali | 25 | Mumbai | 6500.00 |
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| 5 | Hardik | 27 | Bhopal | 8500.00 |
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| 6 | Komal | 22 | MP | 4500.00 |
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| 7 | Muffy | 24 | Indore | 10000.00 |
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The `orders` table:
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| oid | date | customer_id | amount |
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|-----|---------------------|-------------|--------|
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| 102 | 2009-10-08 00:00:00 | 3 | 3000 |
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| 100 | 2009-10-08 00:00:00 | 3 | 1500 |
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| 101 | 2009-11-2 0 00:00:00 | 2 | 1560 |
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| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
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Combining results with `UNION`:
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```sql
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SELECT id, name, amount, date
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FROM customers
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LEFT JOIN orders
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ON customers.id = orders.customer_id
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UNION
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SELECT id, name, amount, date
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FROM customers
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RIGHT JOIN orders
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ON customers.id = orders.customer_id;
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```
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Resulting data set:
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| id | name | amount | date |
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|------|----------|--------|---------------------|
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| 1 | Ramesh | NULL | NULL |
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| 2 | Khilan | 1560 | 2009- 11-20 00:00:00 |
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| 3 | kaushik | 3000 | 2009-10-08 00:00 :00 |
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| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
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| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
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| 5 | Hardik | NULL | NULL |
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| 6 | Komal | NULL | NULL |
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| 7 | Muffy | NULL | NULL |
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## The UNION ALL Clause
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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.
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The requirements for column count, expressions, and data types mirroring across `SELECT` statements remain identical to `UNION`.
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Using the same `customers` and `orders` tables:
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Combining results with `UNION ALL`:
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```sql
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SELECT id, name, amount, date
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FROM customers
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LEFT JOIN orders
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ON customers.id = orders.customer _id
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UNION ALL
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SELECT id, name, amount, date
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FROM customers
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RIGHT JOIN orders
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ON customers.id = orders.customer_id;
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```
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Resulting data set:
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| id | name | amount | date |
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|------|----------|--------|---------------------|
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| 1 | Ramesh | NULL | NULL |
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| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
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| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
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| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
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| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
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| 5 | Hardik | NULL | NULL |
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| 6 | Komal | NULL | NULL |
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| 7 | Muffy | NULL | NULL |
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| 3 | kaushik | 3000 | 2009-10-08 00:00:00 |
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| 3 | kaushik | 1500 | 2009-10-08 00:00:00 |
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| 2 | Khilan | 1560 | 2009-11-20 00:00:00 |
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| 4 | Chaitali | 2060 | 2008-05-20 00:00:00 |
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