Using ‘UNION’ in Laravel Eloquent

Introduction

The UNION SQL operation is an essential tool for database queries that combine results from multiple SELECT statements. It’s invaluable when working with complex data structures, allowing developers to merge rows from different tables or queries. This tutorial will guide you through utilizing the UNION operation within Laravel’s Eloquent ORM.

Understanding UNION in SQL

In understanding how UNION works, it is important to note that it can combine datasets from two or more SELECT statements into a single result set. The key requirement is that each SELECT must have the same number of columns, ideally with similar data types, and the columns must also be in the same order.

The basic syntax for a UNION is as follows:

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;

Basic UNION Query in Laravel

Laravel provides a seamless interface for creating UNION queries through the query builder. Let’s start with a simple example where we have two tables: users and archived_users, and we want to create a combined list of both active and former users.

$first = DB::table('users')->where('active', 1);
$users = DB::table('archived_users')->where('active', 0)
    ->union($first)
    ->get();

This code initiates a query on the users table and assigns it to the variable $first. We then chain the union method to a query on archived_users, resulting in a consolidated collection of both active and inactive users.

Using UNION with Eloquent Models

While the previous example uses the query builder directly, you may also use UNION in conjunction with Laravel’s Eloquent. Assuming we have corresponding models, the implementation might look like the following:

$activeUsers = User::where('active', 1);
$inactiveUsers = ArchivedUser::where('active', 0)
    ->union($activeUsers)
    ->get();

Here, User and ArchivedUser are Eloquent models representing our users and archived_users tables. The code begins by defining the query for active users, and then merges it with the query for inactive users using the union method.

Advanced UNIONS with Ordering and Limitations

More complex UNION queries might require ordering or limiting the final result set. The following example demonstrates this:

$activeUsers = User::where('active', 1)->orderBy('created_at', 'desc');
$inactiveUsers = ArchivedUser::where('active', 0)
    ->union($activeUsers)
    ->orderBy('created_at', 'desc')
    ->limit(10)
    ->get();

This refined query still pulls from active and inactive users, but it now sorts them by their creation date in descending order and limits the result to the latest 10 records.

UNION ALL in Eloquent Queries

By default, UNION removes duplicate rows between the combined sets. However, sometimes you may wish to preserve these duplicates, which is where UNION ALL comes into play. Laravel’s query builder allows you to leverage this operation as well:

$activeUsers = User::where('active', 1);
$inactiveUsers = ArchivedUser::where('active', 0)->unionAll($activeUsers)->get();

In the above implementation, unionAll is used in place of union, and the result will include any overlapping active and inactive users.

UNION with User-Defined Queries

There might be scenarios where a more complex, user-defined sub-query is needed within a UNION. Eloquent allows for this level of complexity:

$customQuery = User::where('active', 1)->select('id', 'name');
$users = DB::query()->fromSub($customQuery, 'active_users')
    ->union($inactiveUsers)
    ->get();

In this case, $customQuery is a user-defined partial query that is later used as a sub-query within the call to fromSub to instantiate a union with $inactiveUsers.

Performance Considerations

While UNION operations are incredibly useful, they can affect performance, particularly with large datasets or when used frequently in a single application. Thus, it’s advised to measure and optimize these queries, potentially leveraging database indexing, query caching, or chunking large outputs for better efficiency.

Conclusion

In this guide, we’ve explored how UNION can be used in Laravel Eloquent to combine data from different tables or queries. Whether you’re a beginner or an expert, understanding how to implement this operation will significantly enhance your ability to interact with your application’s data and provide richer insights through unified data sets.