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Oracle/PLSQL : UNION Operator

Tutorial by:      Date: 2016-04-15 02:24:13

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This Oracle tutorial explains how to use the Oracle UNION operator with syntax and examples.

Description

The Oracle UNION operator is used to combine the result sets of 2 or more Oracle SELECT statements. It removes duplicate rows between the various SELECT statements.

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

Syntax

The syntax for the UNION operator in Oracle/PLSQL 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

The columns or calculations that you wish to retrieve.

tables

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

WHERE conditions

Optional. The conditions that must be met for the records to be selected.

Note

  • There must be same number of expressions in both SELECT statements.

Example - Returns single field

The following is an example of the Oracle 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 order_details;

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

Example - Using ORDER BY

The Oracle UNION operator can use the ORDER BY clause to order the results of the query.

For example:

SELECT supplier_id, supplier_name

FROM suppliers

WHERE supplier_id <= 500

UNION

SELECT company_id, company_name

FROM companies

WHERE company_name = 'Apple'

ORDER BY 2;

In this Oracle UNION operator, 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 Oracle 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 operator 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)

 

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