This page is a quick reference checkpoint for LAG in BigQuery: behavior, syntax rules, edge cases, and a minimal example; plus the official vendor documentation.
LAG lets you look back at a previous row's value within the same partition.
Returns the value of value_expression on a preceding row. - offset must be non-negative; default_expression is used when no row exists at the specified offset. (type and behavior details as documented)
If this behavior feels unintuitive, the tutorial below explains the underlying pattern step-by-step.
LAG(value_expression[, offset [, default_expression ]]) OVER (window_clause)
SELECT division, runner_name, finish_time, LAG(runner_name) OVER (PARTITION BY division ORDER BY finish_time) AS previous_runner FROM race_results;
If you came here to confirm syntax, you’re done. If you came here to get better at window functions, choose your next step.
LAG is part of a bigger window-function pattern. If you want the “why”, start here: Lead Lag
Reading docs is useful. Writing the query correctly under pressure is the skill.
For the authoritative spec, use the vendor docs. This page is the fast “sanity check”.
View BigQuery Documentation →Looking for more functions across all SQL dialects? Visit the full SQL Dialects & Window Functions Documentation.