SQL for Machine Learning

Patterns for extracting ML-ready features from SQL Server — turning normalized relational data into the wide, denormalized, NULL-free datasets that models consume.

When to Use

• Building a feature table or training dataset from application data
• Feature engineering in T-SQL (rolling aggregations, lag features, RFM scoring)
• Encoding categorical variables (one-hot, ordinal, frequency encoding)
• Handling NULLs for ML (imputation strategies in SQL)
• Sampling and train/test splitting from SQL Server
• Exporting large datasets to CSV, Parquet, or pandas
• Preventing data leakage in temporal feature queries
• Writing queries that feed scikit-learn, XGBoost, or PyTorch pipelines

When NOT to use: application schema design (table design, naming conventions, access control), query performance tuning (execution plans, index tuning, wait stats), BI dashboards and summary reports (GROUPING SETS, pivot tables, dashboard queries), or running R/Python inside SQL Server (SQL Server ML Services).

Feature Table Build Workflow

Follow these steps in order. Each step has a validation checkpoint.

1. Set snapshot date — Anchor all features to a fixed @SnapshotDate. Never use GETDATE() inside feature queries.

   • Validate: SELECT @SnapshotDate returns the intended date.

2. Build base entity table — SELECT DISTINCT the entity key into a temp table #Base. One row per entity, primary key only — no features yet.

   SELECT DISTINCT CustomerId
   INTO #Base
   FROM Customers
   WHERE SignupDate < @SnapshotDate;
   

   • Validate: SELECT COUNT(*), COUNT(DISTINCT CustomerId) FROM #Base — counts must match (no duplicates).

3. Join features with temporal bounds — LEFT JOIN each feature set to #Base. Bound every join with AND EventDate <= @SnapshotDate.

   SELECT b.CustomerId,
          DATEDIFF(DAY, f.LastOrder, @SnapshotDate) AS Recency,
          f.OrderCount AS Frequency
   FROM #Base b
   LEFT JOIN FeatureCTE f ON f.CustomerId = b.CustomerId;
   

   • Validate: Row count equals #Base row count. No feature references dates after @SnapshotDate.

4. Impute NULLs — Replace every NULL with an explicit value. Use COALESCE with mean, median, zero, or a sentinel depending on the column semantics.

   • Validate: SELECT SUM(CASE WHEN col IS NULL THEN 1 ELSE 0 END) FROM FeatureTable returns 0 for every column.

5. Encode categoricals — One-hot, ordinal, or frequency encode all non-numeric columns.

   • Validate: SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'FeatureTable' — all columns are numeric types (int, float, decimal, bit).

6. Split train/test — Use hash-based split for cross-sectional data or cutoff-date split for time series.

   • Validate: SELECT SplitLabel, COUNT(*) FROM FeatureTable GROUP BY SplitLabel — verify expected proportions (e.g., 80/20).

7. Export — BCP, pandas.read_sql, or FOR JSON depending on the consumer.

Feature Engineering Taxonomy

Category	Examples	T-SQL tools
Numeric	Raw values, LOG(col + 1) for skewed, ratios, z-scores	LOG(col + 1), SQRT, window AVG/STDEV
Categorical	One-hot encoding, ordinal, frequency	CASE, PIVOT, DENSE_RANK, COUNT ratios
Temporal	Recency, duration, day-of-week	DATEDIFF, DATEPART, DATENAME
Rolling window	7-day sum, 30-day average	SUM/AVG OVER (ROWS BETWEEN ...)
Lag / offset	Previous value, delta from prior	LAG, LEAD
Interaction	Product, ratio of two features	Computed expressions in SELECT
Text signals	Length, keyword presence	LEN, CHARINDEX, PATINDEX
Missingness	Is this value missing?	CASE WHEN col IS NULL THEN 1 ELSE 0 END

Quick Reference: SQL Pattern → ML Concept

ML concept	T-SQL pattern
Recency feature	DATEDIFF(DAY, LastEventDate, @SnapshotDate)
Frequency feature	COUNT(*) OVER (PARTITION BY entity ORDER BY dt ROWS BETWEEN 29 PRECEDING AND CURRENT ROW)
Rolling 7-day revenue	SUM(Amount) OVER (ORDER BY OrderDate ROWS BETWEEN 6 PRECEDING AND CURRENT ROW)
Lag feature (t-1)	LAG(Amount, 1) OVER (PARTITION BY CustomerId ORDER BY OrderDate)
One-hot encode	CASE WHEN Category = 'A' THEN 1 ELSE 0 END AS Category_A
Ordinal encode	DENSE_RANK() OVER (ORDER BY Category)
Frequency encode	COUNT(*) OVER (PARTITION BY Category) * 1.0 / COUNT(*) OVER ()
Log transform	LOG(Amount + 1) — the +1 offset handles zero values; omit only when zeros are impossible
Quantile bucket	NTILE(10) OVER (ORDER BY Score)
Row hash for split	5-step chain: (1) CAST(Id AS NVARCHAR(20)) → (2) HASHBYTES('SHA2_256', ...) → (3) CAST(... AS BINARY(8)) → (4) CAST(... AS BIGINT) → (5) ABS(...) % 10
Mean imputation	COALESCE(col, AVG(col) OVER ())
Median imputation	PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY col) OVER ()
NULL indicator	CASE WHEN col IS NULL THEN 1 ELSE 0 END AS col_is_missing
Min-max scaling	(col - MIN(col) OVER ()) / NULLIF(MAX(col) OVER () - MIN(col) OVER (), 0)
Date bucketing	DATE_BUCKET(WEEK, 1, EventDate) (SQL Server 2022+)
Date truncation	DATETRUNC(MONTH, EventDate) (SQL Server 2022+)
First value in series	FIRST_VALUE(col) OVER (PARTITION BY entity ORDER BY dt ROWS UNBOUNDED PRECEDING)
Rolling volatility	STDEV(col) OVER (PARTITION BY entity ORDER BY dt ROWS BETWEEN 29 PRECEDING AND CURRENT ROW)

Common Mistakes

Mistake	What goes wrong	Fix
Using AVG(col) without understanding NULL exclusion	AVG divides by non-NULL count, not row count — mean is inflated when NULLs represent zeros	Use AVG(COALESCE(col, 0)) when NULLs mean zero
Rolling window with RANGE instead of ROWS	Tied timestamps include extra rows, producing different counts per row	Always use ROWS BETWEEN n PRECEDING AND CURRENT ROW
Joining to future events in feature query	Leaks information that wasn't available at prediction time	Bound all joins with AND EventDate <= @SnapshotDate
Computing global statistics before the train/test split	Mean and variance incorporate test data into training features	Compute statistics only over training rows
Using LAG without checking temporal order	LAG requires ORDER BY — missing it gives non-deterministic offsets	Always include PARTITION BY entity ORDER BY timestamp
Forward-filling sensor NULLs across entity boundaries	PARTITION BY is missing, so last value bleeds across entities	Always PARTITION BY entity_id in forward-fill window
HASHBYTES on NULL key	HASHBYTES(algo, NULL) returns NULL — entity lands in no split	Exclude or handle NULL keys before splitting
UNPIVOT for categorical to indicator columns	UNPIVOT silently drops NULL values	Use CROSS APPLY VALUES to preserve NULLs as zeros
Random split for time-series data	Test set leaks future patterns into training window	Use a cutoff date for time-series train/test split
Including the label column in lag features	LAG(label) leaks ground truth about adjacent rows	Exclude any derivative of the target from feature set

Reference Files

• Feature Engineering — numeric, categorical, temporal, rolling windows, lag, RFM, text
• Sampling and Splitting — TABLESAMPLE, hash-based splits, NTILE k-fold, stratified, time-based
• NULL Imputation — mean/median/mode/forward-fill/constant, missingness indicators
• Export Patterns — BCP, pandas.read_sql, chunked reads, FOR JSON, Parquet, feature stores
• Data Leakage — temporal, target, train/test leakage; prevention patterns in SQL