[10.x] Database Expressions that are conditions

[tpetry]

With #46558 merged, the database expressions could now support more extended use cases: A common thing for database drivers or users is to extend the query builder to support new whereXyz() methods. But these are not database-agnostic.

I've created a new interface (ConditionExpression) to indicate expressions that will be transformed into an entire database condition. These can be passed to where and having:

/** @see https://github.com/tpetry/laravel-query-expressions/blob/main/src/Operator/Comparison/DistinctFrom.php */
$query->where(new DistinctFrom('type', new Value('pending')))

Expressions built on the new interface will be handled differently by WHERE and HAVING. At the moment, an expression passed as a single parameter to where will result in a SQL string that is no longer the intended use-case. By changing the query builder very minimal to handle ConditionExpression different, those conditions can be embedded into the SQL query correctly:

-- expression using the Expression interface
WHERE (type IS DISTINCT FROM 'pending') IS NULL

-- expression using the ConditionExpression interface
WHERE (type IS DISTINCT FROM 'pending')

But this approach is much more compelling if you consider more complex use cases. For example, geographical operations or statistical queries require many transformations on columns and values. These are hard to generalize, and you must fall back to raw parts. Some spaghetti code often insecurely interpolates those raw strings when complex customization is used.

With condition expressions, you can safely composite multiple objects to form the condition you want to have. Complex dynamic query-building logic can be transformed into parsers that return a single object that can be used with Laravel:

$condition = createDBCondition($request);
// returns:
// new LessThan(
//   new GeoDistance('location', new GeoPoint(-39.08677, 58.64465, srid: 4326), srid: 4326),
//   new Value(50),
// );

$query->where(condition);