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4.6 KiB
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125 lines
4.6 KiB
Markdown
# SQL - UNIONS CLAUSE
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The SQL UNION clause/operator is used to combine the results of two or more SELECT statements without returning any duplicate rows.
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- While using this UNION clause, each SELECT statement must have:
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- The same number of columns selected
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- The same number of column expressions
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- The same data type and
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- Have them in the same order
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But they need not have to be in the same length.
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_Example_
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Consider the following two tables.
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Table 1 − customers table is as follows:
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+----+----------+-----+-----------+----------+
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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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+----+----------+-----+-----------+----------+
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Table 2 − orders table is as follows:
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+-----+---------------------+-------------+--------+
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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 | 2009-11-20 00:00:00 | 2 | 1560 |
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| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
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+-----+---------------------+-------------+--------+
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Now, let us join these two tables in our SELECT statement as follows:
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```sql
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SELECT id, name, amount, date
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FROM customer
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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 customer
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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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This would produce the following result:
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### The UNION ALL Clause
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The UNION ALL operator is used to combine the results of two SELECT statements including duplicate rows.
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The same rules that apply to the UNION clause will apply to the UNION ALL operator.
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_Example_ -
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Consider the following two tables:
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* Table 1 − customers table is as follows:
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+----+----------+-----+-----------+----------+
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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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+----+----------+-----+-----------+----------+
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* Table 2 − orders table is as follows:
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+-----+---------------------+-------------+--------+
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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 | 2009-11-20 00:00:00 | 2 | 1560 |
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| 103 | 2008-05-20 00:00:00 | 4 | 2060 |
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+-----+---------------------+-------------+--------+
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Now, let us join these two tables in our SELECT statement as follows :
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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 = order.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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This would produce the following result:
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+------+----------+--------+---------------------+
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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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+------+----------+--------+---------------------+
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Note : **There are two other clauses (i.e., operators), which are like the UNION clause.**
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