DROP VLABEL and DROP ELABEL#

Removes a vertex label or an edge label, and with CASCADE the rows in it.

Synopsis#

DROP VLABEL [ IF EXISTS ] label_name [, ... ] [ CASCADE | RESTRICT ]
DROP ELABEL [ IF EXISTS ] label_name [, ... ] [ CASCADE | RESTRICT ]

bold keywords · italic placeholders you supply · [ ] optional · ... repeatable. See Syntax conventions.

Description#

DROP VLABEL drops a vertex label and DROP ELABEL an edge label; each refuses the other kind. Dropping a label drops the table behind it, along with its id sequence, its indexes and its constraints.

RESTRICT, which is the default, has two things to object to:

  • Rows. A label that still holds vertices or edges is refused with cannot drop <label> because it is not empty. This is stricter than the dependency check PostgreSQL applies to a table, and it catches people out.

  • Children. A label with a child label is refused, and the message names the child.

CASCADE overrides both. It drops a label whatever it holds, and:

  • dropping a vertex label deletes the edges attached to its vertices, whatever labels those edges have. The edge labels themselves survive.

  • dropping an edge label deletes only its own edges. The vertices they connected survive.

The label is looked up in the graph named by graph_path; the name cannot be qualified.

  • The base labels ag_vertex and ag_edge cannot be dropped.

  • The table behind a label cannot be dropped on its own: DROP TABLE on it is refused and points you here.

  • To empty a graph without dropping its labels, use MATCH (n) DETACH DELETE n instead.

Parameters#

IF EXISTS

Issue a notice instead of an error for a label that does not exist. Other names in the same statement are still dropped.

label_name

Name of the label to drop. Several names may be given, separated by commas.

CASCADE

Drop the label along with its rows and its child labels. For a vertex label, also drop the edges attached to its vertices.

RESTRICT

Refuse if the label holds any rows or has a child label. This is the default.

Examples#

The examples build their own graph, tidygraph, rather than using the example dataset, and drop it at the end.

CREATE GRAPH tidygraph;
SET graph_path = tidygraph;
CREATE VLABEL film;
CREATE VLABEL person;
CREATE ELABEL directed;
CREATE (:person {name: 'Denis Villeneuve'})-[:directed]->(:film {title: 'Arrival'});
CREATE VLABEL draft;

Drop an empty label#

DROP VLABEL draft;

DROP VLABEL

Nothing else is reported, because nothing else was affected.

Drop a label that still holds vertices#

film has a vertex in it, so the default refuses:

DROP VLABEL film;

ERROR:  cannot drop film because it is not empty.

CASCADE takes the label, its vertex, and the directed edge that pointed at it:

DROP VLABEL film CASCADE;

DROP VLABEL

MATCH ()-[e:directed]->() RETURN count(e) AS directed_edges;

 directed_edges 
----------------
 0
(1 row)

The edge is gone but its label is not. directed is still there, empty:

\dGl tidygraph.dir*

           List of labels
   Graph   |   Name   | Type | Owner
-----------+----------+------+-------
 tidygraph | directed | edge | agens
(1 row)

Drop a parent label#

CREATE VLABEL work;
CREATE VLABEL documentary INHERITS (work);
DROP VLABEL work;

ERROR:  cannot drop vlabel work because other objects depend on it
DETAIL:  vlabel documentary depends on vlabel work
HINT:  Use DROP ... CASCADE to drop the dependent objects too.

CASCADE names the child it takes with it:

DROP VLABEL work CASCADE;

NOTICE:  drop cascades to vlabel documentary
DROP VLABEL

Drop several labels, skipping the ones that are gone#

CREATE VLABEL a;
CREATE VLABEL b;
DROP VLABEL IF EXISTS a, b, c;

NOTICE:  vlabel "c" does not exist, skipping
DROP VLABEL

Refusals worth recognizing#

Each statement drops one kind of label:

DROP ELABEL person;

ERROR:  DROP ELABEL cannot DROP vertex label

The base labels stay:

DROP VLABEL ag_vertex;

ERROR:  DROP base vertex label is prohibited

And the table cannot be dropped out from under the label:

DROP TABLE tidygraph.person;

ERROR:  cannot drop table tidygraph.person because vlabel person requires it
HINT:  You can drop vlabel person instead.

Finally, drop the graph the examples built:

DROP GRAPH tidygraph CASCADE;

Notes#

An edge whose endpoint vertex is deleted is deleted with it, both here and in DETACH DELETE. start and end are plain graphid columns rather than foreign keys. The graph write path does this, not a referential action on the table.

Note

cypher_allow_unsafe_ddl switches off the kind check and the base-label check, so with it on DROP ELABEL will drop a vertex label and ag_vertex can be dropped. It exists to repair a graph whose catalog is already wrong. Leave it off otherwise.

Compatibility#

ISO/IEC 39075 (GQL)#

GQL has no statement that drops a label. Element types belong to a graph type, which is dropped whole by <drop graph type statement> (§12.7), and dropping one does not delete any data. Graph types are tracked as planned in the GQL conformance matrix.

PostgreSQL#

DROP VLABEL and DROP ELABEL correspond to DROP TABLE, and IF EXISTS, the name list and CASCADE/RESTRICT behave as they do there. One addition has no DROP TABLE counterpart. DROP TABLE does not care how many rows a table has; these statements refuse a label that is not empty unless you write CASCADE. And for a vertex label, CASCADE also deletes edges in other labels. Read CASCADE here as “and the graph data that depends on it”, not only “and the dependent catalog objects”.

See also#