T-SQL (Transact-SQL) is Microsoft's procedural extension to standard SQL. While standard SQL is declarative (you say what you want), T-SQL adds procedural capabilities (you can control how to get it). Think of standard SQL as ordering food at a restaurant - you specify what you want. T-SQL is like having a kitchen where you can control the cooking process step-by-step. This matters because complex business logic often requires conditional logic, loops, and error handling that standard SQL cannot provide.

**1. Variables and Data Types**

Variables store intermediate results and parameters. Declared with DECLARE, assigned with SET or SELECT.

```sql

-- Declare variables

DECLARE @OrderCount INT;

DECLARE @TotalRevenue DECIMAL(10,2);

DECLARE @CustomerName NVARCHAR(100);

-- Assign with SET (single value)

SET @OrderCount = (SELECT COUNT(*) FROM orders WHERE status = 'completed');

-- Assign with SELECT (can set multiple variables)

SELECT @TotalRevenue = SUM(total_amount),

@OrderCount = COUNT(*)

FROM orders

WHERE order_date >= '2024-01-01';

-- Use in queries

SELECT * FROM orders WHERE total_amount > @TotalRevenue / @OrderCount;

```

**Why this matters**: Stripe uses variables to store payment thresholds and fraud detection scores during transaction processing. Variables reduce query complexity and make logic reusable.

**2. Control Flow Statements**

IF/ELSE for conditional logic, WHILE for loops, CASE for inline conditions.

```sql

-- IF/ELSE example

DECLARE @OrderCount INT = (SELECT COUNT(*) FROM orders);

IF @OrderCount > 1000

BEGIN

PRINT 'High volume - enable batch processing';

UPDATE settings SET batch_mode = 1;

END

ELSE IF @OrderCount > 100

BEGIN

PRINT 'Medium volume - standard processing';

UPDATE settings SET batch_mode = 0;

END

ELSE

BEGIN

PRINT 'Low volume - real-time processing';

UPDATE settings SET batch_mode = 0, realtime = 1;

END;

-- WHILE loop example (use sparingly)