SQL: UNION ALL Operator
This SQL tutorial explains how to use the SQL UNION ALL operator with syntax and examples.
Description
The SQL UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It does not remove duplicate rows between the various SELECT statements (all rows are returned).
Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types.
What is the difference between UNION and UNION ALL?
- UNION removes duplicate rows.
- UNION ALL does not remove duplicate rows.
Syntax
The syntax for the UNION ALL operator in SQL is:
SELECT expression1, expression2, ... expression_n FROM tables [WHERE conditions] UNION ALL 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
- The corresponding expressions must have the same data type in the SELECT statements. For example: expression1 must be the same data type in both the first and second SELECT statement.
- See also the UNION operator.
Example - Single Field With Same Name
Let's look at how to use the SQL UNION ALL operator that returns one field. In this simple example, the field in both SELECT statements will have the same name and data type.
For example:
SELECT supplier_id FROM suppliers UNION ALL SELECT supplier_id FROM orders ORDER BY supplier_id;
This SQL UNION ALL example would return the supplier_id multiple times in the result set if that same value appeared in both the suppliers and orders table. The SQL UNION ALL operator does not remove duplicates. If you wish to remove duplicates, try using the UNION operator.
Now, let's explore this example further will some data.
If you had the suppliers table populated with the following records:
supplier_id | supplier_name |
---|---|
1000 | Microsoft |
2000 | Oracle |
3000 | Apple |
4000 | Samsung |
And the orders table populated with the following records:
order_id | order_date | supplier_id |
---|---|---|
1 | 2015-08-01 | 2000 |
2 | 2015-08-01 | 6000 |
3 | 2015-08-02 | 7000 |
4 | 2015-08-03 | 8000 |
And you executed the following UNION ALL statement:
SELECT supplier_id FROM suppliers UNION ALL SELECT supplier_id FROM orders ORDER BY supplier_id;
You would get the following results:
supplier_id |
---|
1000 |
2000 |
2000 |
3000 |
4000 |
6000 |
7000 |
8000 |
As you can see in this example, the UNION ALL has taken all supplier_id values from both the suppliers table as well as the orders table and returned a combined result set. No duplicates were removed as you can see by the supplier_id value of 2000 appearing twice in the result set.
Example - Different Field Names
It is not necessary that the corresponding columns in each SELECT statement have the same name, but they do need to be the same corresponding data types.
When you don't have the same column names between the SELECT statements, it gets a bit tricky, especially when you want to order the results of the query using the ORDER BY clause.
Let's look at how to use the UNION ALL operator with different column names and order the query results.
For example:
SELECT supplier_id, supplier_name FROM suppliers WHERE supplier_id > 2000 UNION ALL SELECT company_id, company_name FROM companies WHERE company_id > 1000 ORDER BY 1;
In this SQL UNION ALL 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_id / company_id in ascending order, as denoted by the ORDER BY 1
. The supplier_id / company_id fields are in position #1 in the result set.
Now, let's explore this example further with data.
If you had the suppliers table populated with the following records:
supplier_id | supplier_name |
---|---|
1000 | Microsoft |
2000 | Oracle |
3000 | Apple |
4000 | Samsung |
And the companies table populated with the following records:
company_id | company_name |
---|---|
1000 | Microsoft |
3000 | Apple |
7000 | Sony |
8000 | IBM |
And you executed the following UNION ALL statement:
SELECT supplier_id, supplier_name FROM suppliers WHERE supplier_id > 2000 UNION ALL SELECT company_id, company_name FROM companies WHERE company_id > 1000 ORDER BY 1;
You would get the following results:
supplier_id | supplier_name |
---|---|
3000 | Apple |
3000 | Apple |
4000 | Samsung |
7000 | Sony |
8000 | IBM |
First, notice that the record with supplier_id of 3000 appears twice in the result set because the UNION ALL query returns all rows and does not remove duplicates.
Second, notice that the column headings in the result set are called supplier_id and supplier_name. This is because these were the column names used in the first SELECT statement in the UNION ALL.
If you had wanted to, you could have aliased the columns as follows:
SELECT supplier_id AS ID_Value, supplier_name AS Name_Value FROM suppliers WHERE supplier_id > 2000 UNION ALL SELECT company_id AS ID_Value, company_name AS Name_Value FROM companies WHERE company_id > 1000 ORDER BY 1;
Now the column headings in the result will be aliased as ID_Value for the first column and Name_Value for the second column.
ID_Value | Name_Value |
---|---|
3000 | Apple |
3000 | Apple |
4000 | Samsung |
7000 | Sony |
8000 | IBM |
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