APIv4 Explicit Joins¶
In a relational database, you often need to work with data from multiple entities (tables) at once. If the simpler notations for implicit join or option transformation don't meet a need, you can use the explicit syntax to indicate exactly how 2 tables should be joined.
Joins or Chaining
There is some overlap between joins and Chaining; in some cases either technique can accomplish the same thing. Because chaining executes a new query for every result, joins are usually more efficient when given a choice between the two.
Specifying a Join¶
An explicit join is defined as an array with the following properties:
EnityName AS alias- the alias can be any string of your choosing; it will be used as an identifier in other clauses (
- Join type (
- LEFT joins make the related entity optional - rows from the main entity are always returned.
- INNER joins make the related entity required - rows from the main entity are only returned if the join is found.
- EXCLUDE only returns rows from the main entity are only returned if the join is not found.
EntityBridge(optional) name of bridge to use (not shown, see following section on EntityBridges)
- One or more
ONclause arrays; each is either a single clause with
[expression, operator, expression], or a conjunction (
NOT) followed by sub-clauses.
// Traditional Syntax: civicrm_api4('Contact', 'get' [ 'join' => [ [ 'Participant AS participants', // Entity & alias 'LEFT', // Type of join NULL, // entity bridge to use ['id', '=', 'participants.contact_id'], // ON clauses... ['participants.status_id:name', '=', "'Attended'"], ], ], ...
Quoting Strings in ON Clauses
Unlike WHERE or HAVING, the ON clause can reference field names on either side of the operator, e.g. 'participants.contact_id'. To specify a string literal like "Attended", place it in a second set of quotation marks (single or double). Note that arrays of strings do not need extra quotes, as in the below example:
The OOP syntax has a sugar
addJoin method which takes the name/alias and side as the first 2 arguments,
then an optional bridge (not shown, see following section on EntityBridges), and a variable number of clauses.
// OOP Syntax: Civi\Api4\Contact::get() ->addJoin( 'Participant AS participants', 'LEFT', NULL, ['id', '=', 'participants.contact_id'], ['participants.status_id:name', 'IN', ['Registered', 'Attended']] ) ...
Specifying 0 Clauses Deprecated
If you pass 0 clauses to the join (e.g.
'join' => [['Participant AS participants', 'LEFT']])
The API will attempt to find an FK between the two entities and automatically add it to the ON clause for you.
Because this can lead to ambiguity and unexpected behavior, it is recommended to always explicitly give the FK fields
in the ON clause of your join, as do the above examples.
GROUP BY and Aggregation¶
The relationship between joined entities is not necessarily 1-1. By default this will cause duplicate rows to be returned
for the main entity. For example, an API call to
Contact.get with a join on
This example groups the results by
Contact.id to return just one row per contact, and aggregates
their email addresses into an array:
civicrm_api4('Contact', 'get', [ 'select' => ['display_name', 'GROUP_CONCAT(email.email) AS emails'], 'join' => [ ['Email AS email', 'LEFT', ['id', '=', 'email.contact_id']], ], 'groupBy' => ['id'], ...
Using an EntityBridge¶
An EntityBridge is a small table whose primary purpose is to link two other entities together.
They exist because MySQL cannot directly link a row in one table to multiple rows in another.
The EntityBridge solves this by storing a row for every join. For example, a single activity can
have more than one target contact, so the
ActivityContact stores a row for every
Contact linked to an
APIv4 allows you to join two entities through an EntityBridge with a single join clause. Just add the name of the bridge API in front of the join clauses:
civicrm_api('Activity', 'get', [ 'join' => [ [ "Contact AS target_contacts", // Entity & alias "LEFT", // Type of join "ActivityContact", // EntityBridge name ['id', "=", 'target_contacts.activity_id'], // ON clauses... ['target_contacts.record_type_id:name', '=', '"Activity Targets"'], ], ], ...
Notice that the ON clause specifies joining directly to the Contact entity, even though the fields
record_type_id actually belong to
ActivityContact. This is because behind the scenes,
the API combines the two, allowing you to treat the
Contact entity as if it also has fields
belonging to the