The ELEMENT_NUMBER function can only be used for iterator variables that are defined in the ONE ROW PER VERTEX or ONE ROW PER STEP clause in a SQL graph query.

The function references an iterator variable and returns the sequential element number that the iterator variable currently binds to. Since paths always start with a vertex and alternate between vertices and edges, the first element is a vertex with element number 1, the second element is an edge with element number 2, the third element is a vertex with element number 3, and so on. For example, consider the following path: v1 -[e1]-> v2 -[e2]-> v3. Then the element numbers are generated from left to right as shown:

v1:1, e1:2, v2:3, e2:4, v3:5

Therefore, vertices always have odd element numbers while edges have even element numbers.

If ONE ROW PER STEP is specified and the path is empty (that is, only has a single vertex and no edges), then ELEMENT_NUMBER returns NULL when the iterator edge variable or the second iterator vertex variable is referenced. Note that empty paths result in single steps in which only the first iterator (vertex) variable is bound.

Also, note the following:

• ELEMENT_NUMBER can be used only in the COLUMNS clause.
• ELEMENT_NUMBER function can only be used if ONE ROW PER VERTEX or ONE ROW PER STEP is specified in the SQL graph query.
• ELEMENT_NUMBER cannot reference any type of variable other than an iterator variable.

For instance, consider the following sample query. The query uses ONE ROW PER STEP and the ELEMENT_NUMBER function returns the element number of the graph element that is bound by the iterator variable v.

SELECT *
FROM GRAPH_TABLE ( g
       MATCH (a) ->{0,2} ONE ROW PER STEP (v,e,v2) 
       COLUMNS (ELEMENT_NUMBER(v) AS elementNumber)
)

See Example 5-20 for more information.