12.6. Foreach

The FOREACH clause is used to update data within a collection, whether components of a path, or result of aggregation.

Collections and paths are key concepts in Cypher. To use them for updating data, you can use the FOREACH construct. It allows you to do updating commands on elements in a collection — a path, or a collection created by aggregation.

The identifier context inside of the foreach parenthesis is separate from the one outside it. This means that if you CREATE a node identifier inside of a FOREACH, you will not be able to use it outside of the foreach statement, unless you match to find it.

Inside of the FOREACH parentheses, you can do any of the updating commands — CREATE, CREATE UNIQUE, MERGE, DELETE, and FOREACH.

If you want to execute an additional MATCH for each element in a collection then UNWIND (see Section 10.6, “Unwind”) would be a more appropriate command.

Figure 12.2. Data for the examples

Mark all nodes along a path

This query will set the property marked to true on all nodes along a path.


MATCH p =(begin)-[*]->(END )
WHERE begin.name='A' AND END .name='D'
FOREACH (n IN nodes(p)| SET n.marked = TRUE )

Nothing is returned from this query, but four properties are set.


Properties set: 4

(empty result)

Try this query live create (_0 {`name`:"D"}) create (_1 {`name`:"A"}) create (_2 {`name`:"B"}) create (_3 {`name`:"C"}) create _1-[:`KNOWS`]->_2 create _2-[:`KNOWS`]->_3 create _3-[:`KNOWS`]->_0 match p = (begin)-[*]->(end) where begin.name='A' and end.name='D' foreach(n in nodes(p) | set n.marked = true)