What is the difference between UNION and UNION ALL?

UNION combines results from two queries and removes duplicate rows (like DISTINCT). UNION ALL combines results without removing duplicates, which is faster. Use UNION ALL when you know there are no duplicates or when duplicates are acceptable.

Do UNION queries require the same number of columns?

Yes. Both SELECT statements in a UNION must have the same number of columns, and corresponding columns must have compatible data types. The column names from the first SELECT are used in the result.

Can I use ORDER BY with UNION?

Yes, but ORDER BY must appear at the end of the entire UNION statement, not inside each individual SELECT. Example: SELECT ... UNION SELECT ... ORDER BY column_name.

SQL UNION

SQL > Advanced SQL > Union

The purpose of the SQL UNION query is to combine the results of two queries together while removing duplicates. In other words, when using UNION, only unique values are returned (similar to SELECT DISTINCT).

Key Takeaway: UNION stacks the results of two SELECT queries on top of each other and removes duplicate rows. Both queries must return the same number of columns with compatible data types.

Syntax

The syntax of UNION in SQL is as follows:

[SQL Statement 1]
UNION
[SQL Statement 2];

The columns selected in [SQL Statement 1] and [SQL Statement 2] need to be of the same data type for UNION to work.

Example

We use the following tables for our example.

Table Store_Information

Store_Name	Sales	Txn_Date
Los Angeles	1500	Jan-05-1999
San Diego	250	Jan-07-1999
Los Angeles	300	Jan-08-1999
Boston	700	Jan-08-1999

Table Internet_Sales

Txn_Date	Sales
Jan-07-1999	250
Jan-10-1999	535
Jan-11-1999	320
Jan-12-1999	750

To find all the dates where there is a sales transaction, we use the following SQL statement:

SELECT Txn_Date FROM Store_Information
UNION
SELECT Txn_Date FROM Internet_Sales;

Result:

Txn_Date

Jan-05-1999

Jan-07-1999

Jan-08-1999

Jan-10-1999

Jan-11-1999

Jan-12-1999

Please note that if we type "SELECT DISTINCT Txn_Date" for either or both of the SQL statement, we will get the same result set.

Frequently Asked Questions

When should I use UNION vs JOIN?

Use UNION to stack rows from two queries vertically (same columns, different rows). Use JOIN to combine columns horizontally from two tables. They solve different problems: UNION expands the row count, JOIN expands the column count.

Why does Jan-07-1999 appear only once in the result even though it's in both tables?

Because UNION removes duplicates. Jan-07-1999 appears in Store_Information AND Internet_Sales, but UNION returns it only once. Use UNION ALL to see it twice.

Can I UNION more than two queries?

Yes. You can chain multiple UNIONs: SELECT ... UNION SELECT ... UNION SELECT .... All queries must have the same column structure.

Does UNION preserve the original row order?

No. The order of rows in a UNION result is not guaranteed unless you add an ORDER BY clause at the end of the full UNION statement.

Next: SQL UNION ALL

This page was last updated on March 19, 2026.

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