Sql Query - Using Order By in UNION

sql

join

best-practices

dataframe

byAnton Shumikhin·Sep 16, 2024

Here's your MVP (Minimal Viable Product) of a UNION query with an ORDER BY clause. It's as simple as wrapping your UNION in a subquery and then hitting it with ORDER BY at the end:

SELECT * FROM (
    SELECT column FROM table1  -- Extracting wisdom from Table1
    UNION
    SELECT column FROM table2  -- Uncorking knowledge from Table2
) AS subquery  -- Voila! They're one big happy family now
ORDER BY column;  -- Clean and tidy. Just the way we like it!

This does the ordering of your data on subquery, presenting you with a collective, assorted result set.

Addressing discrepancies in column names

In cases where the columns you're fetching bear different names across tables, you can bring them on the same page using aliases. This ensures consistency across columns and keeps the ORDER BY directive applicable to all the result sets:

SELECT column1 AS myColumn FROM table1  -- column1 prefers pseudonyms. Let's call it myColumn
UNION
SELECT column2 AS myColumn FROM table2  -- column2 jumps on the alias bandwagon. myColumn it is!
ORDER BY myColumn;  -- You see? There's nothing a good 'ol alias can't sort out!

The UNION ALL alternative for preserving duplicates

Want to keep all the duplicate records safe and secure? UNION ALL plays the knight in shining armor. It merges the data without removing any duplicates:

SELECT column FROM table1  -- Picking the apples
UNION ALL  -- None shall be left behind! Not even the duplicates
SELECT column FROM table2  -- Picking the oranges
ORDER BY column;  -- And it's in perfect order. Just knowing it's there feels good!

Employing ORDER BY within individual UNION components

For those with a penchant for deep clean organizing can carry out sort operations even before the UNION, by using the ORDER BY clause in the individual SELECT statements:

SELECT column FROM table1
ORDER BY column  -- OCD Alert! Sorting before uniting
UNION
SELECT column FROM table2
ORDER BY column;  -- Another one bites the dust(of disorder)

This way, you achieve a preliminary sort before bringing the results together. Especially valuable for the big data enthusiasts among us!

Navigating the MS Access syntax maze

For the brave souls venturing into the realm of the MS Access Jet database engine, knowing the slightly different syntax is key to keeping your queries running smoothly:

SELECT * FROM (
    SELECT column FROM table1  -- Same column, different alias. Call it multitasking!
    UNION
    SELECT column FROM table2
) AS mySubquery  -- Announcing the gloriously united data
ORDER BY mySubquery.column;  -- Setting the house in order

Redefining the subquery as AS subquery and fully qualifying the column name in the ORDER BY clause gets the job done.

Testing the integrity of your ORDER BY in UNION queries

Test your ORDER BY clause within your UNION queries like so:

Whip up test cases with predefined outcomes.
Make a comparison of the sorted results from each SELECT statement versus the collective UNION query.
Verify the consistency of your query results across diverse database offerings like MySQL, SQL Server, and PostgreSQL.