SQL
Conditional Aggregation, Pivot and Unpivot
Turning rows into columns and back again with portable SQL. Conditional aggregation, the PIVOT operator where it exists, and unpivoting with UNION ALL...
#Aggregation
Summarising many rows into one value.
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SQL
OVER, PARTITION BY and Window ORDER BY
Window functions compute across related rows without collapsing them. Learn the OVER clause, PARTITION BY, window ORDER BY, and how they differ from G...
SQL
Multi Column Grouping and Grouping Rules
Grouping by several columns, grouping by expressions, and the totals features - ROLLUP, CUBE and GROUPING SETS - that add subtotal rows.
SQL
HAVING, and WHERE vs HAVING
HAVING filters groups after aggregation, WHERE filters rows before it. Learn when each applies, why using both is usually correct, and which is faster...
SQL
GROUP BY Fundamentals
GROUP BY collapses rows into one row per distinct key. Learn the single rule that governs the select list and how NULLs are grouped.
SQL
Aggregates, NULL and Aggregate Expressions
Aggregating an expression rather than a column: weighted totals, conditional counts and the difference between counting rows and counting facts.
SQL
COUNT, SUM, AVG, MIN and MAX
The five aggregate functions that collapse many rows into one value, the three forms of COUNT, and what each one does with NULL.