What Is ST_UNION in BigQuery?

ST_UNION is commonly used to aggregate spatial data, such as combining district, regional, or territorial boundaries for analysis. By merging shapes, analysts can simplify datasets, reduce redundancy, and perform advanced geospatial operations like urban planning, regional mapping, and demographic visualization, all within SQL-based workflows.

Syntax of ST_UNION in BigQuery

The ST_UNION function combines two or more geometries into one. It can be used with columns in a table or directly within expressions.

Syntax:

ST_UNION(geometry_expression1[, geometry_expression2, ...])

or

SELECT ST_UNION(geometry_column) FROM table_name;

Each geometry expression must contain valid geography data. The function returns a single geometry representing the merged area, which can be visualized or further queried.

Benefits of Using ST_UNION in BigQuery

ST_UNION provides significant advantages for analysts and GIS professionals working with spatial data.

• Aggregates boundaries efficiently: Combines multiple geographic features into one logical unit.
• Simplifies geospatial datasets: Reduces data complexity for mapping and visualization.
• Supports regional analysis: Enables urban planning, zoning, or area-based calculations.
• Enhances analytical accuracy: Produces unified geometries for consistent reporting.
• Optimizes workflows: Streamlines multi-region queries for improved performance.

Limitations and Challenges of ST_UNION in BigQuery

The ST_UNION function, while essential for combining geographic shapes, isn’t without its constraints. 

Large datasets, invalid geometries, or overlapping boundaries can introduce performance issues and data inaccuracies if not properly managed.

• Invalid or overlapping geometries: Overlapping or malformed shapes can cause the function to return null results or fail entirely.
• Performance degradation: Running ST_UNION on extensive or complex data slows query performance and increases costs.
• Precision and rounding errors: Merging highly detailed geometries can distort boundaries or lose spatial accuracy.
• High memory usage: Processing numerous polygons can exhaust computational resources and delay execution.
• Data validation needs: Analysts must clean and validate geometries beforehand to ensure consistent and reliable outputs.

Best Practices for Using ST_UNION in BigQuery

Following best practices ensures that the merged geometries are both accurate and resource-efficient, especially when working with large or complex spatial datasets.

• Validate geometries first: Use ST_ISVALID() to check for invalid or self-intersecting geometries.
• Simplify complex shapes: Apply ST_SIMPLIFY() to reduce geometric complexity before merging.
• Aggregate logically: Group data by relevant attributes (e.g., region, zone) before applying ST_UNION.
• Monitor performance: Run smaller test queries to measure compute time before scaling.
• Store results efficiently: Save merged geometries in a separate table for reuse in future spatial analyses.

 Following these steps helps maintain accuracy, improve efficiency, and prevent query slowdowns during large-scale geographic operations.

Real-World Applications of ST_UNION in BigQuery

ST_UNION is widely used across industries for spatial data analysis and location-based decision-making.

• Urban planning: Combine district boundaries to analyze land use or zoning efficiency.
• Marketing and sales: Merge territories to identify high-value regions for targeted campaigns.
• Environmental studies: Consolidate protected areas to assess conservation coverage.
• Transportation analytics: Unite route segments to study infrastructure density.
• Retail network optimization: Group store catchment areas to evaluate overlap or market potential.

 In each use case, ST_UNION enables efficient geographic aggregation and supports more meaningful spatial insights.

Learn More About the ST_UNION Function in BigQuery

The ST_UNION function is a core tool for spatial aggregation in BigQuery, and understanding its relationships with other geospatial functions enhances its usefulness. Pairing it with functions like ST_INTERSECTS, ST_BOUNDARY, and ST_AREA enables deeper geographic analysis—such as identifying overlaps, calculating total coverage, or merging complex boundaries. 

Exploring these functions together helps analysts design efficient, accurate workflows for mapping, zoning, and location-based analytics directly within SQL-driven environments.

Combine Geometries Efficiently with OWOX Data Marts

OWOX Data Marts lets analysts structure and reuse SQL-based geospatial logic across teams and reports. With governed, version-controlled environments, you can merge, query, and publish geospatial datasets automatically. Analysts save time by defining logic once, while business users access unified data across tools like Google Sheets or Looker Studio.