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The Islands T-SQL Challenge: Sequencing Islands

Itzik Ben-Gan has an interesting challenge for us:

The challenge is as follows:

Assuming partitioning based on the column grp and ordering based on the column ord, compute sequential row numbers starting with 1 within each consecutive group of rows with the same value in the val column. Following is the desired result for the given small set of sample data:

Click through for an example of this, a pair of “classic” solutions, and Itzik’s solution.