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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:

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:

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