Authoring data quality checks

A check is one assertion about one warehouse table or view. It compiles to a count-only HogQL query and passes when it finds zero failing rows — the same semantics as dbt test. Failing rows are never stored; only counts and the compiled query are, so to see the offending rows you re-run the stored query yourself.

row_count is the exception. It passes when the observed count is within its configured min/max bounds, so its failed_row_count comes back null and its stored query returns that single count, not offending rows. Read the observed count to judge it rather than looking for matched rows.

Reads go through SQL (system.information_schema.data_quality_*); writes and runs go through the data-quality MCP tools.

Before you write anything: look

Two queries save you from the two most common mistakes — duplicating a check, and checking a column that doesn't exist.