NTILE distributes the rows in an ordered partition into a specified number of groups. The groups are numbered, starting at one.

The following example splits the set of rows in each partition (vertical) into 4 groups in the order of the query latency, and returns the group number for each row.

The Image vertical has 3 rows, thus it has 3 groups.

The Web vertical has 6 rows, the two extra rows are distributed to the first two groups. That's why there are 2 rows in group 1 and group 2, and only 1 row in group 3 and group 4.

@result =
        NTILE(4) OVER(PARTITION BY Vertical ORDER BY Latency) AS Quartile
    FROM @querylog;

The results:

Query Latency Vertical Quartile
Banana 300 Image 1
Cherry 300 Image 2
Durian 500 Image 3
Apple 100 Web 1
Fig 200 Web 1
Papaya 200 Web 2
Fig 300 Web 2
Cherry 400 Web 3
Durian 500 Web 4

NTILE takes a parameter ("numgroups"). Numgroups is a positive int or long constant expression that specifies the number of groups into which each partition must be divided.

If the number of rows in the partition is evenly divisible by numgroups then the groups will have equal size.

If the number of rows in a partition is not divisible by numgroups, this will cause groups of two sizes that differ by one member. Larger groups come before smaller groups in the order specified by the OVER clause.

For example:

  • 100 rows divided into 4 groups: [25, 25, 25, 25]
  • 102 rows divided into 4 groups: [26, 26, 25, 25]

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