This is to document and/or study tasks or transformations that seemingly "should" be easy or intuitive in SQL, but is not, or is not commonly implemented by vendors and/or standardized.

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* Top N of groups, such as top 5 wage earners in each department.

* Converting columns to rows and rows to columns. Example: cross-tab queries.

* Provide global or column-specific substitutions for null cells in outer joins.

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This page differs from SqlFlaws in that it's more about carrying out typical domain tasks rather than language and systems design. Although, there is indeed some overlap.