SQL: UNION Query
The SQL UNION query allows you to combine the result sets of 2 or more SQL SELECT statements. It removes duplicate rows between the various SELECT statements.
Each SQL SELECT statement within the UNION query must have the same number of fields in the result sets with similar data types.
The syntax for the SQL UNION query is:
select field1, field2, . field_n from tables UNION select field1, field2, . field_n from tables;
SQL UNION Query - Returns single field example
The following is an example of the SQL UNION query 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 query example, if a supplier_id appeared in both the suppliers and orders table, it would appear once in your result set. The SQL UNION query removes duplicates. If you do not wish to remove duplicates, try using the SQL UNION ALL query.
SQL UNION Query - Using SQL ORDER BY Clause example
The SQL UNION query 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 query, since the column names are different between the two SQL SELECT statements, it is more advantageous to reference the columns in the SQL 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 SQL COUNT function which is an aggregate function, we'd recommend using an SQL 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 SQL UNION query allows you to perform an SQL COUNT based on one set of criteria.
trunc(last_updated_date) <= trunc(sysdate-13)
As well as perform an SQL COUNT based on another set of criteria.
trunc(last_updated_date) > trunc(sysdate-13)