SQL: UNION Query

The UNION query allows you to combine the result sets of 2 or more "select" queries. It removes duplicate rows between the various "select" statements.

Each SQL statement within the UNION query must have the same number of fields in the result sets with similar data types.

The syntax for a UNION query is:

select field1, field2, . field_n
from tables
UNION
select field1, field2, . field_n
from tables;


Example #1

The following is an example of a UNION query:

select supplier_id
from suppliers
UNION
select supplier_id
from orders;

In this example, if a supplier_id appeared in both the suppliers and orders table, it would appear once in your result set. The UNION removes duplicates.

Example #2 - With ORDER BY Clause

The following is a UNION query that uses an ORDER BY clause:

select supplier_id, supplier_name
from suppliers
where supplier_id > 2000
UNION
select company_id, company_name
from companies
where company_id > 1000
ORDER BY 2;

Since the column names are different between the two "select" statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".

The supplier_name / company_name fields are in position #2 in the result set.

Frequently Asked Questions

Question: I need to compare two dates and return the count of a field based on the date values. For example, I have a date field in a table called last updated date. I have to check if trunc(last_updated_date >= trun(sysdate-13).

Answer: Since you are using the COUNT function which is an aggregate function, we'd recommend using a UNION query. For example, you could try the following:

SELECT a.code as Code, a.name as Name, count(b.Ncode)
FROM cdmaster a, nmmaster b
WHERE a.code = b.code
and a.status = 1
and b.status = 1
and b.Ncode <> 'a10'
and trunc(last_updated_date) <= trunc(sysdate-13)
group by a.code, a.name
UNION
SELECT a.code as Code, a.name as Name, count(b.Ncode)
FROM cdmaster a, nmmaster b
WHERE a.code = b.code
and a.status = 1
and b.status = 1
and b.Ncode <> 'a10'
and trunc(last_updated_date) > trunc(sysdate-13)
group by a.code, a.name;

The UNION query allows you to perform a COUNT based on one set of criteria.

trunc(last_updated_date) <= trunc(sysdate-13)

As well as perform a COUNT based on another set of criteria.

trunc(last_updated_date) > trunc(sysdate-13)