UNION with Order: Mastering the Art of Combining and Sorting SQL Results

This explanation details the challenges of using ORDER BY with UNION and provides solutions for achieving the desired order.

Challenges:

1. Single ORDER BY Clause:
2. Unpredictable Order:

Solutions:

Single ORDER BY at the End:

• Place all your SELECT statements inside parentheses, followed by a single UNION or UNION ALL clause.
• Add the desired ORDER BY clause after the final UNION statement. This clause will apply to the entire combined result set.

Example:

SELECT name, age
FROM customers
WHERE city = 'New York'
ORDER BY age

UNION

SELECT name, age
FROM customers
WHERE city = 'Los Angeles'
ORDER BY age;

Explanation:

• The two SELECT statements each select name and age from the customers table, filtering by city and ordering by age within each statement (these individual orderings are ignored).
• The UNION clause combines the results.
• The final ORDER BY age applies to the entire combined set, sorting all rows by age.

Subqueries with ORDER BY:

• Use subqueries with internal ORDER BY clauses for each desired order.
• Combine the subqueries using UNION or UNION ALL.

(SELECT name, age
FROM customers
WHERE city = 'New York'
ORDER BY age)

UNION ALL

(SELECT name, age
FROM customers
WHERE city = 'Los Angeles'
ORDER BY age DESC);

• Each subquery selects name and age from the customers table, filtered by city and ordered by age (ascending in the first, descending in the second).
• The UNION ALL clause combines both subquery results without removing duplicates.

Related Issues:

• Duplicates with UNION ALL: Using UNION ALL in the previous example will include duplicate rows if both cities have customers with the same name and age. Consider using UNION if duplicates are not desired, but note that it removes duplicates even across different cities.