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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 SQL UNION ALL operator 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.
conditions
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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