SQL UNION operator

Posted by : Glen Sajori Sabtu, 27 September 2014

Description

The SQL UNION operator is used to combine the result sets of 2 or more SELECT statements. It removes duplicate rows between the various SELECT statements. Each SELECT statement within the UNION must have the same number of fields in the result sets with similar data types.

Syntax

The syntax for the SQL UNION operator is:

SELECT expression1, expression2, ... expression_n
FROM tables
WHERE conditions
UNION
SELECT expression1, expression2, ... expression_n
FROM tables
WHERE conditions;

Parameters or Arguments

expression1, expression2, expression_n are the columns or calculations that you wish to retrieve.

tables are the tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

conditions are conditions that must be met for the records to be selected.

Note

There must be same number of expressions in both SELECT statements.
See also the UNION ALL operator.

Example - Return single field

The following is an example of the SQL UNION operator that returns one field from multiple SELECT statements (and both fields have the same data type):

SELECT supplier_id
FROM suppliers
UNION
SELECT supplier_id
FROM orders;

In this SQL UNION operator example, if a supplier_id appeared in both the suppliers and orders table, it would appear once in your result set. The SQL UNION operator removes duplicates. If you do not wish to remove duplicates, try using the UNION ALL operator.

Example - Using SQL ORDER BY Clause

The SQL UNION operator can use the SQL ORDER BY clause to order the results of the query.
For example:

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;

In this SQL UNION example, 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 >= trunc(sysdate-13).

Answer: Since you are using the COUNT function which is an aggregate function, we'd recommend using the Oracle UNION operator. 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 Oracle UNION 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)

Source : techonthenet.com

{ 1 comment... read them below or add one }

obat vitiligoJuli 15, 2016 2:49 PM
aihh mantap dah, makasih banyak infonya, jgn lupa kunjungi balik ya gan, mampir lah :D http://acemaxs31.com/obat-vitiligo/
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