Understanding the Use of UNION with ORDER BY in MySQL

In the world of databases, MySQL is one of the popular relational database management systems. MySQL offers a variety of powerful SQL commands that allow you to retrieve, manipulate, and manage data within tables.

One useful command is UNION, which is used to combine the results of multiple queries into a single set of data.

The UNION command is a tool that enables us to merge the results of two or more queries into a single data set. These queries should produce results with the same number of columns and compatible data types.

UNION automatically eliminates duplicates, meaning if there are identical rows in two query results, only one row will be included in the final result. For example, consider the following case:

SELECT column1 FROM table1
UNION
SELECT column1 FROM table2;

In the above example, we retrieve data from column1 in table1 and combine it with data from column1 in table2. The final result will contain all the unique values from both of these tables.

UNION vs. UNION ALL

In the use of UNION, the final result will only include unique values, whereas UNION ALL will include all rows, including duplicates. The choice between them depends on your needs.

Combining UNION with ORDER BY

When you want to sort the final result obtained from UNION, you can use the ORDER BY command. This allows you to sort the data from the UNION result based on a specific column.

It's important to note that using ORDER BY on UNION will affect the entire combined result, not just one query. Here is an example of using UNION with ORDER BY:

SELECT column1 FROM table1
UNION
SELECT column1 FROM table2
ORDER BY column1 ASC;

In this example, we retrieve data from column1 in table1 and combine it with data from column1 in table2, and then we sort the final result based on column1. The final result will include all unique values from both tables and will be sorted based on column1.

You can replace column1 with the appropriate column name you want to use for sorting, and you can also use ASC (ascending) or DESC (descending) to control the sorting direction.

Note that ORDER BY is always placed after UNION. If you want to sort data before it is combined, you can use a Sub Query.

ORDER BY before UNION

There are times when you need to sort data before performing a UNION. There are two ways to do this, using a Sub Query or using MAX/MIN.

Here's an example of using a Sub Query to sort data before using UNION:

SELECT * FROM
(SELECT column1 FROM table1 ORDER BY column1 ASC LIMIT 1) AS data1
UNION
SELECT * FROM
(SELECT column1 FROM table2 ORDER BY column1 DESC LIMIT 1) AS data2

Here's an example of using MAX/MIN to sort data before using UNION:

SELECT MAX(column1) FROM table1
UNION
SELECT MIN(column1) FROM table2

UNION with ORDER BY can be used to combine and sort data from multiple queries in MySQL. This provides you with greater control over the final results you get from merging data.

Normally, ORDER BY is done after combining data with UNION. However, you can use a Sub Query or MAX/MIN to sort data before combining it.

Remember to understand the differences between UNION and UNION ALL and how to combine them with ORDER BY based on your needs. We hope this article helps you understand these concepts in MySQL database.