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SQL
CASE WHEN — Conditional Values
Inline if/else inside a SELECT, ORDER BY, or aggregation. The Swiss-army knife for converting raw column values into labels or buckets.
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SQL
Window Function vs N+1 Query
Don't fetch a list, then loop in code to count each parent's children. Use a window function or a single GROUP BY — turn 1,001 queries into 1.
SQL
Conditional Aggregation (FILTER / CASE)
Count or sum subsets of rows in a single GROUP BY pass. PostgreSQL has the cleaner `FILTER` clause; everyone else uses `SUM(CASE WHEN ...)`.
SQL
STRING_AGG / GROUP_CONCAT
Concatenate values across a group into a single delimited string. PostgreSQL/MSSQL use `STRING_AGG`; MySQL uses `GROUP_CONCAT`; SQLite has both.
SQL
GROUP BY — Aggregation Basics
`GROUP BY` collapses rows into buckets; the SELECT list must be aggregates OR grouped columns. The most common reporting pattern in SQL.
SQL
ROLLUP — Subtotals and Grand Totals
`GROUP BY ROLLUP` adds subtotal rows (with NULL for the rolled-up columns) plus a grand total. Drop into reports without writing UNIONs by hand.
SQL
Pivot Without PIVOT (Conditional Aggregation)
Most databases don't have a real `PIVOT` keyword (SQL Server does). The portable answer is conditional aggregation — `SUM(CASE WHEN ...) AS col` for each pivoted value.