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.
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Converting columns to rows and rows to columns. Example: cross-tab queries.
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Provide global or column-specific substitutions for null cells in outer joins.
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.