Elgg 1 is introducing a new (to elgg at least) way of executing arbitrary database queries. Basically, SQL is abstracted away into a class that can be used to construct multiple different types of query through a defined interface.

This provides two main advantages:

  1. The application programmer does not have to write the SQL themselves.
  2. It becomes considerably easier to change database back ends later down the line.

In addition, we can add free stuff like access controls etc.

From the documentation:

Query provides a framework to construct complex queries in a safer environment.

The usage of this class depends on the type of query you are executing, but the basic idea is to construct a query out of pluggable classes.

Once constructed SQL can be generated using the toString method, this should happen automatically if you pass the Query object to get_data or similar.

To construct a query, create a new Query() object and begin populating it with the various classes that define the various aspects of the query.


  • You do not have to specify things in any particular order, provided you specify all required components.
  • With database tables you do not have to specify your db prefix, this will be added automatically.
  • When constructing your query keep an eye on the error log – any problems will get spit out here. Note also that __toString won’t let you throw Exceptions (!!!) so these are caught and echoed to the log instead.

Here is an example of a select statement:

// Construct the query
$query = new Query();

// Say which table we're interested in
$query->addTable(new TableQueryComponent("entities"));

// What fields are we interested in
$query->addSelectField(new SelectFieldQueryComponent("entities","*"));

// Add access control (Default access control uses default fields on entities table.
// Note that it will error without something specified here!
$query->setAccessControl(new AccessControlQueryComponent());

// Set a limit and offset, may be omitted.
$query->setLimitAndOffset(new LimitOffsetQueryComponent(10,0));

// Specify the order, may be omitted
$query->setOrder(new OrderQueryComponent("entities", "subtype", "desc"));

// Construct a where query
// This demonstrates a WhereSet which lets you have sub wheres, a
// WhereStatic which lets you compare a table field against a value and a
// Where which lets you compare a table/field with another table/field.
new WhereSetQueryComponent(
new WhereStaticQueryComponent("entities", "subtype","=", 1),
new WhereQueryComponent("entities","subtype","=", "entities", "subtype")


Constructing a query out of classes in this way provides a lot of scope for expanding the functionality within a common framework, as well as providing a structured way of constructing complicated database queries.

However, if it looks too complicated, you can use about 99% of Query‘s functionality by using the subclass SimpleQuery, which provides methods for performing much of the standard functionality.