Self-Join vs Window Function

Concepts covered: sqlSelfJoin

At this level, the aliasing and join syntax is the floor. The interesting decisions are in how you constrain the join: the predicate that determines which pairs survive, the inequality that deduplicates symmetric pairs, and the additional conditions that bound the time or value window. Each of those is a deliberate choice; sloppy choices ship as performance bugs or wrong row counts. The canonical self-join with all three constraints Three constraints in the ON clause. The first bounds the join to pairs that share a customer; without it, every pair of orders across the entire table is a candidate. The second is the directional inequality (o2.order_time > o1.order_time); it eliminates self-pairs and ensures each unordered pair appears only once. The third is the upper bound on the time windo

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