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UNION and UNION ALL let you merge the results of two or more independent queries into a single result set. Both queries can target different files, different paths within the same file, or different filter conditions.

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UNION vs UNION ALL

ClauseDuplicate rows
UNIONRemoved (hash-based deduplication)
UNION ALLKept

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Column count requirement

The number of selected columns must match across all combined queries. If they do not match, FiQueLa throws a QueryLogicException. 
// These column counts must be equal
$query1->select('name', 'price');   // 2 columns
$query2->select('name', 'price');   // 2 columns — OK
Queries using SELECT * skip this validation.

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Basic usage

use FQL\Enum\Operator;
use FQL\Stream\Json;

$stream = Json::open('./data/products.json');

$cheapProducts = $stream->query()
    ->select('id', 'name', 'price')
    ->from('data.products')
    ->where('price', Operator::LESS_THAN_OR_EQUAL, 100);

$expensiveProducts = $stream->query()
    ->select('id', 'name', 'price')
    ->from('data.products')
    ->where('price', Operator::GREATER_THAN_OR_EQUAL, 400);

// UNION — removes duplicate rows
$results = $cheapProducts->union($expensiveProducts)->execute();

// UNION ALL — keeps all rows including duplicates
$results = $cheapProducts->unionAll($expensiveProducts)->execute();

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Chaining multiple unions

You can chain as many unions as needed. Each union executes its own independent pipeline. 
use FQL\Enum\Operator;
use FQL\Stream\Json;

$stream = Json::open('./data/products.json');

$q1 = $stream->query()
    ->select('id', 'name', 'price')
    ->from('data.products')
    ->where('id', Operator::EQUAL, 1);

$q2 = $stream->query()
    ->select('id', 'name', 'price')
    ->from('data.products')
    ->where('id', Operator::EQUAL, 3);

$q3 = $stream->query()
    ->select('id', 'name', 'price')
    ->from('data.products')
    ->where('id', Operator::EQUAL, 5);

$results = $q1
    ->union($q2)
    ->unionAll($q3)
    ->execute();

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How UNION interacts with EXPLAIN

When you run EXPLAIN or EXPLAIN ANALYZE on a query that includes a union, each union branch reports its own sub-phases in the output. Single union produces phases like: 
stream → where → union_stream → union_where → union
Multiple unions index each branch: 
stream → where → union_1_stream → union_1_where → union_1
                 union_2_stream → union_2_where → union_2
See EXPLAIN and EXPLAIN ANALYZE for the full column reference.
Each union sub-query executes its own full pipeline — FROM → WHERE → GROUP BY → SELECT → etc. Clauses like ORDER BY and LIMIT on the main query apply to the final merged result, not to individual branches.