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SQL · PRACTICAL GUIDE

SQL LAG and LEAD

Compare a row with the previous or next event.

Practice SQLLearning path

What this solves

Measure time between consecutive scans. This pattern is useful when the business question is clear but a generic tutorial is too abstract.

The rule

LAG reads a prior row inside a window without a self join.

SELECT EventTime, LAG(EventTime) OVER(ORDER BY EventTime) PrevTime
FROM Events;

Real-work checklist

  1. Define the business key and expected grain before writing the formula, query or markup.
  2. Test the pattern on a small known sample where you can verify the answer manually.
  3. Check missing values, duplicates and data types before trusting a large result.
  4. Scale to the real file or database only after the logic is proven.

Common mistake

Define PARTITION BY when separate machines or orders must not influence each other.

Try it next

PracticeChange the example and verify the result.Use a toolMove from a small example to real work.Related contentFind guides, errors and scenarios on this topic.