SAMPLE¶

The SAMPLE clause takes samples evenly in the result set and returns the specified amount of data.

SAMPLE can be used in GO statements only. The syntax is as follows:

<go_statement> SAMPLE <sample_list>;

sample_list is a list. Elements in the list must be natural numbers, and the number of elements must be the same as the maximum number of STEPS in the GO statement. The following takes GO 1 TO 3 STEPS FROM "A" OVER * SAMPLE <sample_list> as an example to introduce this usage of SAMPLE in detail.

The list sample_list must contain 3 natural numbers, such as GO 1 TO 3 STEPS FROM "A" OVER * SAMPLE [1,2,4].
1 in SAMPLE [1,2,4] means that the system automatically selects 1 edge to continue traversal in the first step. 2 means to select 2 edges to continue traversal in the second step. 4 indicates that 4 edges are selected to continue traversal in the third step. If there is no matched edge in a certain step or the number of matched edges is less than the specified number, the actual number will be returned.
Because GO 1 TO 3 STEPS means to return all the traversal results from the first to third steps, all the red edges and their source and destination vertices in the figure below will be matched by this GO statement. And the yellow edges represent there is no path selected when the GO statement traverses. If it is not GO 1 TO 3 STEPS but GO 3 STEPS, it will only match the red edges of the third step and the vertices at both ends.

SAMPLE in GO

In the basketballplayer dataset, the example is as follows:

nebula> GO 3 STEPS FROM "player100" \
        OVER * \
        YIELD properties($$).name AS NAME, properties($$).age AS Age \
        SAMPLE [1,2,3];
+-----------------+----------+
| NAME            | Age      |
+-----------------+----------+
| "Tony Parker"   | 36       |
| "Manu Ginobili" | 41       |
| "Spurs"         | __NULL__ |
+-----------------+----------+

nebula> GO 1 TO 3 STEPS FROM "player100" \
        OVER * \
        YIELD properties($$).name AS NAME, properties($$).age AS Age \
        SAMPLE [2,2,2];
+-----------------+----------+
| NAME            | Age      |
+-----------------+----------+
| "Manu Ginobili" | 41       |
| "Spurs"         | __NULL__ |
| "Tim Duncan"    | 42       |
| "Spurs"         | __NULL__ |
| "Manu Ginobili" | 41       |
| "Spurs"         | __NULL__ |
+-----------------+----------+

Last update: February 19, 2024