DST Query Skill

DST Query Skill

Purpose

Execute SQL queries to analyze DST data stored in DuckDB. This is the core skill for data analysis - enabling filtering, aggregation, joins, and extracting insights from stored statistical data.

DST Data Patterns

Handling Suppressed Values

DST uses ".." for suppressed/​confidential data. Must handle before numeric operations.

Filter approach:

SELECT * FROM dst_table
WHERE INDHOLD != '..'  -- Filter out suppressed

Safe casting approach:

SELECT
  TID,
  CASE
    WHEN INDHOLD != '..' THEN CAST(INDHOLD AS INTEGER)
    ELSE NULL
  END as value
FROM dst_table

Using helpers:

from scripts.db.helpers import safe_numeric_cast

query = f"SELECT TID, {safe_numeric_cast('INDHOLD')} as value FROM dst_table"

Aggregate Codes

DST uses special codes for totals/​aggregates:

• "TOT" - Total
• "I alt" - Total (Danish)
• "Drivmidler i alt" - All fuel types
• "IALT" - Total (alternative)

Filter them out when analyzing details:

SELECT * FROM dst_table
WHERE fuel_type NOT IN ('Drivmidler i alt', 'I alt')
  AND gender NOT IN ('TOT', 'IALT')

Time Format Handling

DST time codes as strings don't sort chronologically:

• "2024K1" - Quarter 1, 2024
• "2024M01" - January 2024
• "2024" - Year 2024

Extract for proper sorting:

-- For quarters
SELECT
  TID,
  CAST(SUBSTRING(TID, 1, 4) AS INTEGER) as year,
  CAST(SUBSTRING(TID, 6, 1) AS INTEGER) as quarter
FROM dst_table
WHERE TID LIKE '%K%'
ORDER BY year, quarter

When to Use

• User asks analytical questions about the data
• Need to filter or aggregate data
• Joining multiple DST tables
• Extracting specific insights or trends
• Computing custom statistics
• Exploring table structure and contents (use table summary)

Table Summary

Purpose

Get a quick overview of table structure and statistics before detailed querying.

Usage

python scripts/​db/​table_summary.py --table-id <TABLE_ID>

When to Use

• Before writing complex queries
• Understanding table structure
• Checking available columns
• Seeing sample data
• Getting quick statistics

Output Includes

• Record count
• Column names and types
• Sample rows (first 5)
• Statistics for numeric columns (min, max, avg, median)
• Distinct value counts
• NULL counts
• Top values for categorical columns

Running SQL Queries

Basic Usage

Execute a SQL query:

python scripts/​db/​query_data.py --sql "<QUERY>"

Output Formats

Table format (default - console-friendly):

--format table

JSON format (for programmatic use):

--format json

CSV format (for exports):

--format csv

Save to File

Save query results:

--output <file>

Safety Limit

Add automatic LIMIT:

--limit 100

Table Naming Convention

All DST tables in DuckDB follow this pattern:

• Format: dst_{table_id} (lowercase)
• Example: FOLK1A → dst_folk1a
• Example: AUP01 → dst_aup01

Important: Always use lowercase in queries.

Data Format in DuckDB

Tables stored from DST API use these conventions:

• Separator: Data was fetched as semicolon-separated CSV (;)
• Encoding: UTF-8 with BOM (handled automatically)
• Column names: Based on variable IDs from tableinfo
• Value codes: Exact codes from DST (e.g., "000", "101", "2024K1")
• Data types: DuckDB infers types (usually strings for codes, numeric for values)

Common Query Patterns

1. Explore Data

SELECT * FROM dst_folk1a LIMIT 10

2. Count Records

SELECT COUNT(*) FROM dst_folk1a

3. Check Column Structure

-- See what columns exist
DESCRIBE dst_folk1a;

-- Or use table summary (recommended)
-- python scripts/​db/​table_summary.py --table-id FOLK1A

4. Aggregation

SELECT region, SUM(population) as total_pop
FROM dst_folk1a
GROUP BY region
ORDER BY total_pop DESC

5. Time Series Analysis

-- Note: Time codes from DST (e.g., "2024K1" for Q1 2024)
SELECT tid, value
FROM dst_folk1a
WHERE område = '000'  -- Whole country
ORDER BY tid

6. Filtering with DST Codes

-- Use exact codes from tableinfo
SELECT *
FROM dst_folk1a
WHERE tid LIKE '2024%'  -- All 2024 periods
  AND område IN ('000', '101')  -- Denmark and Copenhagen
  AND køn IN ('1', '2')  -- Men and women (not 'TOT')

7. Multiple Aggregations

SELECT
  område,
  COUNT(*) as record_count,
  AVG(CAST(indhold AS DOUBLE)) as avg_value,
  MAX(CAST(indhold AS DOUBLE)) as max_value
FROM dst_folk1a
GROUP BY område

8. Join Tables

SELECT
  a.område,
  a.indhold as population,
  b.indhold as employment
FROM dst_folk1a a
JOIN dst_aup01 b ON a.område = b.område AND a.tid = b.tid
WHERE a.tid = '2024K1'

9. Percentages

SELECT
  område,
  CAST(indhold AS DOUBLE) as value,
  100.0 * CAST(indhold AS DOUBLE) / SUM(CAST(indhold AS DOUBLE)) OVER () as percentage
FROM dst_folk1a
WHERE tid = '2024K1' AND køn = 'TOT'

10. Latest Period Analysis

-- Find most recent quarter
WITH latest AS (
  SELECT MAX(tid) as max_tid FROM dst_folk1a
)
SELECT område, SUM(CAST(indhold AS DOUBLE)) as total
FROM dst_folk1a
WHERE tid = (SELECT max_tid FROM latest)
GROUP BY område
ORDER BY total DESC

Best Practices

Query Development

1. Start with table summary to understand structure
2. Use LIMIT for exploratory queries
3. Build incrementally - test small queries first
4. Check record counts before expensive operations

Performance

• Use WHERE clauses to filter early
• Add indexes if querying repeatedly (advanced)
• Aggregate before joining when possible
• Be mindful of large result sets

Safety

• Queries are READ-ONLY (SELECT only)
• Cannot modify data (no INSERT/​UPDATE/​DELETE)
• Cannot alter schema (no DROP/​CREATE/​ALTER)
• Script validates queries before execution

Data Quality

• Handle NULL values explicitly
• Use COALESCE for NULL handling
• Verify data types before operations
• Check for duplicates if unexpected

Understanding Data Freshness

Before analyzing, check when data was last updated:

SELECT table_id, last_updated, row_count
FROM dst_metadata
WHERE table_id = 'FOLK1A'

Recommendations:

• Check freshness before major analysis
• Re-fetch if data is stale (use dst-check-freshness skill)
• Note DST update frequency varies by table
• Some tables update quarterly, others monthly or annually

Examples

Example 1: Get table summary

python scripts/​db/​table_summary.py --table-id FOLK1A

Example 2: Simple exploration

python scripts/​db/​query_data.py --sql "SELECT * FROM dst_folk1a LIMIT 5"

Example 3: Aggregation by year

python scripts/​db/​query_data.py --sql "SELECT year, SUM(population) as total FROM dst_folk1a GROUP BY year ORDER BY year"

Example 4: Regional analysis

python scripts/​db/​query_data.py --sql "SELECT region, AVG(value) as avg_val FROM dst_folk1a WHERE year >= 2020 GROUP BY region"

Example 5: Export to CSV

python scripts/​db/​query_data.py --sql "SELECT * FROM dst_folk1a" --format csv --output results.csv

Example 6: JSON output

python scripts/​db/​query_data.py --sql "SELECT * FROM dst_folk1a LIMIT 100" --format json --output data.json

Example 7: With safety limit

python scripts/​db/​query_data.py --sql "SELECT * FROM dst_folk1a" --limit 1000

Advanced Queries

Window Functions

SELECT
  year,
  value,
  LAG(value) OVER (ORDER BY year) as prev_year_value,
  value - LAG(value) OVER (ORDER BY year) as change
FROM dst_folk1a
WHERE region = '000'
ORDER BY year

Pivoting Data

SELECT
  region,
  MAX(CASE WHEN year = 2023 THEN value END) as val_2023,
  MAX(CASE WHEN year = 2024 THEN value END) as val_2024
FROM dst_folk1a
GROUP BY region

Percentiles

SELECT
  PERCENTILE_CONT(0.25) WITHIN GROUP (ORDER BY value) as p25,
  PERCENTILE_CONT(0.50) WITHIN GROUP (ORDER BY value) as median,
  PERCENTILE_CONT(0.75) WITHIN GROUP (ORDER BY value) as p75
FROM dst_folk1a

Complex Filtering

SELECT *
FROM dst_folk1a
WHERE year BETWEEN 2020 AND 2024
  AND region IN (SELECT DISTINCT region FROM dst_aup01 WHERE employment > 1000)
  AND value IS NOT NULL
ORDER BY value DESC
LIMIT 100

Tips

Before Querying

• Run table summary first to see structure
• Check column names and types (often Danish: område, tid, køn, etc.)
• Review sample data to understand value formats
• Verify table has data and check last_updated in dst_metadata
• Note: Column names are lowercase variable IDs from DST

Working with DST Data Codes

• Time codes: "2024K1" (quarterly), "2024M01" (monthly), "2024" (annual)
• Geographic codes: "000" (whole country), "101" (Copenhagen), etc.
• Aggregate codes: "TOT", "IALT" often represent totals
• Use LIKE: For pattern matching time periods: tid LIKE '2024%'
• Cast when needed: Value columns may be strings: CAST(indhold AS DOUBLE)

Query Writing

• Use table aliases for clarity (a, b, etc.)
• Format SQL for readability
• Comment complex queries
• Test with LIMIT first
• Handle Danish characters properly (æ, ø, å)

Analysis Workflow

1. Understand: Get table summary
2. Check freshness: Query dst_metadata for last_updated
3. Explore: Simple SELECT with LIMIT
4. Filter: Add WHERE clauses with exact DST codes
5. Aggregate: Use GROUP BY (cast numeric columns first)
6. Refine: Add ORDER BY, calculations
7. Export: Save final results

Performance Tips

• Filter first, aggregate second
• Use specific columns, not SELECT *
• Add LIMIT for large tables
• Use indexes on commonly filtered columns (advanced)
• Consider creating views for repeated queries (advanced)
• Cache results locally if running same query multiple times

Troubleshooting

"Table not found"

• Verify table exists: python scripts/​db/​list_tables.py (dst-list-tables skill)
• Check table name is lowercase
• Ensure format: dst_{table_id}
• Example: FOLK1A becomes dst_folk1a

"Column not found"

• Run table summary to see actual column names
• Column names are lowercase DST variable IDs
• Common columns: område (region), tid (time), køn (gender), indhold (value)
• Check spelling including Danish characters (æ, ø, å)
• Verify column exists in that specific table

Data Type Issues

• Value columns often stored as strings (e.g., indhold)
• Cast to numeric for calculations: CAST(indhold AS DOUBLE)
• Time codes are strings: use LIKE for patterns
• Don't assume numeric types without checking

Unexpected Results

• Empty results: Check if data was actually fetched for that table
• Wrong aggregations: Verify you're filtering out 'TOT' codes if needed
• Time ordering issues: Time codes as strings may not sort chronologically
  • Solution: Extract year/​quarter or use CASE statements
• Duplicate rows: Table may have multiple dimensions - check GROUP BY

Large Result Sets

• Add LIMIT clause for exploration
• Use aggregation to reduce rows
• Export to file instead of console: --output results.csv
• Check row count first: SELECT COUNT(*) FROM table

Slow Queries

• Check WHERE filters are effective
• Filter by indexed columns (primary keys)
• Simplify joins
• Reduce columns selected
• Check data size with COUNT first
• Avoid SELECT * on large tables

Query Syntax Errors

• Verify SQL syntax (DuckDB follows PostgreSQL conventions)
• Check quotes: use single quotes for string literals
• Danish characters: ensure UTF-8 encoding
• Test simple version first
• Review error message carefully

Character Encoding Issues

• DuckDB handles UTF-8 automatically
• If seeing odd characters, verify terminal encoding
• CSV exports preserve Danish characters (æ, ø, å)

Common Workflows

Workflow 1: Explore New Table

# 1. Get summary
python scripts/​db/​table_summary.py --table-id FOLK1A

# 2. See sample data
python scripts/​db/​query_data.py --sql "SELECT * FROM dst_folk1a LIMIT 5"

# 3. Check record count
python scripts/​db/​query_data.py --sql "SELECT COUNT(*) FROM dst_folk1a"

# 4. Explore key dimensions
python scripts/​db/​query_data.py --sql "SELECT DISTINCT region FROM dst_folk1a"

Workflow 2: Trend Analysis

# 1. Get summary statistics
python scripts/​db/​table_summary.py --table-id FOLK1A

# 2. Query time series
python scripts/​db/​query_data.py --sql "SELECT year, SUM(value) as total FROM dst_folk1a GROUP BY year ORDER BY year"

# 3. Calculate growth
python scripts/​db/​query_data.py --sql "SELECT year, value, value - LAG(value) OVER (ORDER BY year) as growth FROM dst_folk1a WHERE region = '000'"

# 4. Export results
python scripts/​db/​query_data.py --sql "..." --format csv --output analysis.csv

Workflow 3: Compare Regions

# 1. Get regional breakdown
python scripts/​db/​query_data.py --sql "SELECT region, COUNT(*) as records, AVG(value) as avg_value FROM dst_folk1a GROUP BY region ORDER BY avg_value DESC"

# 2. Top regions
python scripts/​db/​query_data.py --sql "SELECT region, SUM(value) as total FROM dst_folk1a WHERE year = 2024 GROUP BY region ORDER BY total DESC LIMIT 10"

# 3. Compare specific regions
python scripts/​db/​query_data.py --sql "SELECT year, region, value FROM dst_folk1a WHERE region IN ('000', '101', '147') ORDER BY year, region"

SQL Reference

Useful DuckDB Functions

Aggregation:

• COUNT, SUM, AVG, MIN, MAX
• MEDIAN, STDDEV, VARIANCE
• STRING_AGG (concatenate strings)

String Functions:

• UPPER, LOWER, TRIM
• SUBSTRING, CONCAT
• LIKE, ILIKE (case-insensitive)

Date Functions:

• CURRENT_DATE, CURRENT_TIMESTAMP
• DATE_DIFF, DATE_ADD
• EXTRACT (year, month, day)

Window Functions:

• ROW_NUMBER, RANK, DENSE_RANK
• LAG, LEAD
• FIRST_VALUE, LAST_VALUE

Conditional:

• CASE WHEN ... THEN ... END
• COALESCE (handle NULLs)
• NULLIF

Best Practices Summary

1. Always start with table summary
2. Use LIMIT during development
3. Build queries incrementally
4. Handle NULLs explicitly
5. Use clear aliases and formatting
6. Test before running on full dataset
7. Export results for further analysis
8. Document complex queries