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---
name: data-querying
description: Write and verify SQL queries with BigQuery. Use when executing bq commands, writing SQL queries, or including query results in documents.
---
# Data Querying
## Verification Requirement
All queries MUST be executed with `bq` and verified before inclusion. Never include unverified queries or fabricated data.
## Query Process
1. **Dry run**: Validate syntax and check cost
```bash
bq query --project_id=<PROJECT_ID> --use_legacy_sql=false --dry_run "SELECT ..."
```
Cost: ~$5/TB. <1GB is light, 2GB+ needs optimization.
2. **Execute**: Run and confirm results
```bash
bq query --project_id=<PROJECT_ID> --use_legacy_sql=false --format=csv "SELECT ..."
```
3. **Learn**: Study existing queries in project docs for patterns.
## Query Design
- Specify exact date ranges
- Filter partitioned tables by partition key
- Avoid correlated subqueries (use JOINs/CTEs)
- Filter early with CTEs before joining large tables
## Documentation
1. Execute exact query with `bq`
2. Copy real output only
3. State time period and what query measures
