advanced-patterns

name	advanced-patterns
description	Advanced T-SQL patterns and techniques for SQL Server. Use this skill when: (1) User needs help with CTEs or recursive queries, (2) User asks about APPLY operator, (3) User wants MERGE or OUTPUT clause help, (4) User works with temporal tables, (5) User needs In-Memory OLTP guidance, (6) User asks about advanced grouping (ROLLUP, CUBE, GROUPING SETS).

Advanced T-SQL Patterns

Advanced techniques for complex SQL Server scenarios.

Quick Reference

Pattern Selection Guide

Task	Pattern
Hierarchical data	Recursive CTE
Top N per group	ROW_NUMBER + CTE
Correlated subquery alternative	CROSS/OUTER APPLY
Upsert (insert or update)	MERGE
Capture modified rows	OUTPUT clause
Historical data tracking	Temporal tables
High-throughput OLTP	In-Memory OLTP
Multiple aggregation levels	ROLLUP/CUBE/GROUPING SETS

Common Table Expressions (CTEs)

Basic CTE

WITH RecentOrders AS (
    SELECT CustomerID, OrderDate, Amount
    FROM Orders
    WHERE OrderDate >= DATEADD(month, -3, GETDATE())
)
SELECT c.CustomerName, r.Amount
FROM Customers c
JOIN RecentOrders r ON c.CustomerID = r.CustomerID;

Multiple CTEs

WITH
Sales AS (
    SELECT ProductID, SUM(Amount) AS TotalSales FROM Orders GROUP BY ProductID
),
Inventory AS (
    SELECT ProductID, SUM(Quantity) AS TotalInventory FROM Stock GROUP BY ProductID
)
SELECT p.ProductName, s.TotalSales, i.TotalInventory
FROM Products p
LEFT JOIN Sales s ON p.ProductID = s.ProductID
LEFT JOIN Inventory i ON p.ProductID = i.ProductID;

Recursive CTE (Hierarchies)

WITH OrgChart AS (
    -- Anchor: Top-level (no manager)
    SELECT EmployeeID, Name, ManagerID, 0 AS Level,
           CAST(Name AS VARCHAR(1000)) AS Path
    FROM Employees
    WHERE ManagerID IS NULL

    UNION ALL

    -- Recursive: Subordinates
    SELECT e.EmployeeID, e.Name, e.ManagerID, oc.Level + 1,
           CAST(oc.Path + ' > ' + e.Name AS VARCHAR(1000))
    FROM Employees e
    JOIN OrgChart oc ON e.ManagerID = oc.EmployeeID
)
SELECT * FROM OrgChart
OPTION (MAXRECURSION 100);  -- Default is 100, max is 32767

CTE for Deleting Duplicates

WITH Duplicates AS (
    SELECT *,
           ROW_NUMBER() OVER (
               PARTITION BY Email
               ORDER BY CreatedDate DESC
           ) AS RowNum
    FROM Users
)
DELETE FROM Duplicates WHERE RowNum > 1;

APPLY Operator

CROSS APPLY (Inner Join Behavior)

-- Top 3 orders per customer
SELECT c.CustomerID, c.Name, o.OrderID, o.Amount
FROM Customers c
CROSS APPLY (
    SELECT TOP 3 OrderID, Amount
    FROM Orders
    WHERE CustomerID = c.CustomerID
    ORDER BY OrderDate DESC
) o;

OUTER APPLY (Left Join Behavior)

-- Last order per customer (including customers with no orders)
SELECT c.CustomerID, c.Name, o.LastOrderDate, o.LastOrderAmount
FROM Customers c
OUTER APPLY (
    SELECT TOP 1 OrderDate AS LastOrderDate, Amount AS LastOrderAmount
    FROM Orders
    WHERE CustomerID = c.CustomerID
    ORDER BY OrderDate DESC
) o;

APPLY with Table-Valued Function

-- Call function for each row
SELECT c.CustomerID, f.MonthlyTotal, f.OrderCount
FROM Customers c
CROSS APPLY dbo.GetCustomerMonthlyStats(c.CustomerID) f;

APPLY to Unpivot Columns

-- Transform columns to rows
SELECT ID, AttributeName, AttributeValue
FROM Products
CROSS APPLY (
    VALUES
        ('Color', Color),
        ('Size', Size),
        ('Weight', CAST(Weight AS VARCHAR))
) AS Unpivoted(AttributeName, AttributeValue)
WHERE AttributeValue IS NOT NULL;

MERGE Statement

Basic Upsert

MERGE INTO TargetTable AS t
USING SourceTable AS s
ON t.ID = s.ID
WHEN MATCHED THEN
    UPDATE SET t.Name = s.Name, t.Value = s.Value, t.UpdatedAt = GETDATE()
WHEN NOT MATCHED BY TARGET THEN
    INSERT (ID, Name, Value, CreatedAt)
    VALUES (s.ID, s.Name, s.Value, GETDATE())
WHEN NOT MATCHED BY SOURCE THEN
    DELETE
OUTPUT $action, inserted.*, deleted.*;

MERGE with Conditions

MERGE INTO Products AS t
USING StagingProducts AS s
ON t.ProductID = s.ProductID
WHEN MATCHED AND s.Price <> t.Price THEN
    UPDATE SET t.Price = s.Price, t.LastModified = GETDATE()
WHEN MATCHED AND s.Discontinued = 1 THEN
    DELETE
WHEN NOT MATCHED THEN
    INSERT (ProductID, Name, Price) VALUES (s.ProductID, s.Name, s.Price);

OUTPUT Clause

Capture Inserted Rows

DECLARE @InsertedRows TABLE (ID INT, Name VARCHAR(100));

INSERT INTO Customers (Name, Email)
OUTPUT inserted.CustomerID, inserted.Name INTO @InsertedRows
VALUES ('John', '[e-mail]'), ('Jane', '[e-mail]');

SELECT * FROM @InsertedRows;

Capture Updated Rows (Before and After)

UPDATE Products
SET Price = Price * 1.1
OUTPUT deleted.ProductID, deleted.Price AS OldPrice, inserted.Price AS NewPrice
WHERE Category = 'Electronics';

Capture Deleted Rows

DELETE FROM ExpiredOrders
OUTPUT deleted.*
INTO OrderArchive
WHERE ExpiryDate < DATEADD(year, -1, GETDATE());

Advanced Grouping

ROLLUP (Hierarchical Subtotals)

SELECT
    COALESCE(Region, 'Total') AS Region,
    COALESCE(Product, 'All Products') AS Product,
    SUM(Sales) AS TotalSales
FROM SalesData
GROUP BY ROLLUP (Region, Product);
-- Groups: (Region, Product), (Region), ()

CUBE (All Combinations)

SELECT Region, Product, SUM(Sales) AS TotalSales
FROM SalesData
GROUP BY CUBE (Region, Product);
-- Groups: (Region, Product), (Region), (Product), ()

GROUPING SETS (Custom Combinations)

SELECT Region, Product, Year, SUM(Sales)
FROM SalesData
GROUP BY GROUPING SETS (
    (Region, Product),
    (Region, Year),
    (Product),
    ()
);

Identify Grouping Level

SELECT
    CASE WHEN GROUPING(Region) = 1 THEN 'All' ELSE Region END AS Region,
    CASE WHEN GROUPING(Product) = 1 THEN 'All' ELSE Product END AS Product,
    SUM(Sales) AS TotalSales,
    GROUPING_ID(Region, Product) AS GroupLevel
    -- GroupLevel: 0 = both, 1 = Product rolled up, 2 = Region rolled up, 3 = both
FROM SalesData
GROUP BY ROLLUP (Region, Product);

Temporal Tables (SQL 2016+)

Create System-Versioned Table

CREATE TABLE Products (
    ProductID INT PRIMARY KEY,
    Name NVARCHAR(100),
    Price DECIMAL(18,2),
    ValidFrom DATETIME2 GENERATED ALWAYS AS ROW START,
    ValidTo DATETIME2 GENERATED ALWAYS AS ROW END,
    PERIOD FOR SYSTEM_TIME (ValidFrom, ValidTo)
)
WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.ProductsHistory));

Query Historical Data

-- Point in time
SELECT * FROM Products
FOR SYSTEM_TIME AS OF '2024-01-01 12:00:00';

-- Time range
SELECT * FROM Products
FOR SYSTEM_TIME BETWEEN '2024-01-01' AND '2024-06-30';

-- All history
SELECT * FROM Products
FOR SYSTEM_TIME ALL;

In-Memory OLTP

Create Memory-Optimized Table

-- First add filegroup
ALTER DATABASE YourDB
ADD FILEGROUP MemOptFG CONTAINS MEMORY_OPTIMIZED_DATA;

ALTER DATABASE YourDB
ADD FILE (NAME = 'MemOptFile', FILENAME = 'C:\Data\MemOpt') TO FILEGROUP MemOptFG;

-- Create table
CREATE TABLE OrdersMemOpt (
    OrderID INT NOT NULL PRIMARY KEY NONCLUSTERED HASH WITH (BUCKET_COUNT = 1000000),
    CustomerID INT NOT NULL INDEX IX_Customer NONCLUSTERED HASH WITH (BUCKET_COUNT = 100000),
    OrderDate DATETIME2 NOT NULL,
    Amount DECIMAL(18,2) NOT NULL,
    INDEX IX_Date NONCLUSTERED (OrderDate)
) WITH (MEMORY_OPTIMIZED = ON, DURABILITY = SCHEMA_AND_DATA);

Natively Compiled Procedure

CREATE PROCEDURE InsertOrderFast
    @CustomerID INT,
    @Amount DECIMAL(18,2)
WITH NATIVE_COMPILATION, SCHEMABINDING
AS
BEGIN ATOMIC WITH (TRANSACTION ISOLATION LEVEL = SNAPSHOT, LANGUAGE = N'English')
    INSERT INTO dbo.OrdersMemOpt (OrderID, CustomerID, OrderDate, Amount)
    VALUES (NEXT VALUE FOR dbo.OrderSeq, @CustomerID, SYSDATETIME(), @Amount);
END;

Table-Valued Constructor

VALUES as Table

SELECT * FROM (
    VALUES
        (1, 'Apple', 1.50),
        (2, 'Banana', 0.75),
        (3, 'Orange', 2.00)
) AS Products(ID, Name, Price);

Use in MERGE

MERGE INTO Products AS t
USING (VALUES
    (1, 'Apple', 1.60),
    (2, 'Banana', 0.80)
) AS s(ID, Name, Price)
ON t.ID = s.ID
WHEN MATCHED THEN UPDATE SET Price = s.Price
WHEN NOT MATCHED THEN INSERT VALUES (s.ID, s.Name, s.Price);

Sequences

Create and Use Sequence

CREATE SEQUENCE OrderSeq
    AS INT START WITH 1 INCREMENT BY 1;

-- Get next value
SELECT NEXT VALUE FOR OrderSeq;

-- Use in INSERT
INSERT INTO Orders (OrderID, CustomerID)
VALUES (NEXT VALUE FOR OrderSeq, @CustomerID);

-- Use as default
ALTER TABLE Orders
ADD CONSTRAINT DF_OrderID DEFAULT NEXT VALUE FOR OrderSeq FOR OrderID;