Learn how to use the SQL UNION operator with syntax and examples.
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.
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;
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.
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.
The SQL UNION operator can use the SQL ORDER BY clause to order the results of the query.
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.
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)