Conditional aggregation with CASE

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Wrapping CASE inside SUM (or COUNT, AVG) is called conditional aggregation - it lets you pivot or filter within an aggregate without a subquery. For Alice: shipped order = 100, pending order = 0 (ELSE). SUM = 100. For Bob: shipped order = 300. SUM = 300. A common variant: COUNT(CASE WHEN status = 'shipped' THEN 1 END) counts shipped rows per customer.