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Creating Aggregate Functions

To create an aggregate function, you need to create an implementation of the AggregateFunction or SummarizableAggregateFunction interface.

Interfaces​

Aggregate function Aggregate function

AggregateFunction Interface​

namespace Rekalogika\Analytics\Contracts\Summary;

use Rekalogika\Analytics\Contracts\Context\SummaryQueryContext;

interface AggregateFunction
{
    public function getAggregateToResultExpression(
        string $inputExpression,
        SummaryQueryContext $context,
    ): string;
}

Implement AggregateFunction if the function does not read any data from the source directly, but gets its input from other aggregate functions. Example: the Average aggregate function, it takes the sum and count from other aggregate functions and uses them to calculate the average.

SummarizableAggregateFunction Interface​

namespace Rekalogika\Analytics\Contracts\Summary;

use Rekalogika\Analytics\Contracts\Context\SourceQueryContext;

interface SummarizableAggregateFunction extends AggregateFunction
{
    public function getSourceToAggregateExpression(
        SourceQueryContext $context,
    ): string;

    public function getAggregateToAggregateExpression(
        string $inputExpression,
    ): string;

    /**
     * @return list<string>
     */
    public function getInvolvedProperties(): array;
}

Implement SummarizableAggregateFunction if the function reads data from the source entity. You need to supply these logics using DQL expressions:

  • getSourceToAggregateExpression() - How to transform a field from multiple source entities into a single aggregate value.
  • getAggregateToAggregateExpression() - How to combine multiple aggregate values into a single aggregate value.
  • getAggregateToResultExpression() - How to transform an aggregate value into the final result value.

Division Consideration​

When creating an aggregate function that involves division, you should consider the case when the denominator is zero. To avoid division by zero, use the DQL expression NULLIF(<denominator>, 0). This will return NULL if the denominator is zero, which is a safe way to handle division in DQL.