Using Common Table Expressions (CTEs) in T-SQL

Introduction to Common Table Expressions (CTEs)

Common Table Expressions (CTEs) are a powerful feature in T-SQL that allow you to create temporary result sets that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. They enhance query organization, readability, and maintainability.

Why This Matters

CTEs improve query structure by breaking down complex SQL statements into manageable segments. They can also enable recursion and help simplify repetitive code.

Step-by-Step Guide to Using CTEs

Basic Syntax

The syntax for a CTE is straightforward:

WITH CTE_Name AS (<     SELECT column1, column2<     FROM table_name<     WHERE condition< )< SELECT * FROM CTE_Name;

Example 1: Simple CTE

In this example, we’ll create a CTE to find employees with salaries greater than a specific amount:

WITH HighEarners AS (<     SELECT EmployeeID, FirstName, LastName, Salary<     FROM Employees<     WHERE Salary > 50000< )< SELECT * FROM HighEarners;

Example 2: CTE with Recursive Query

CTEs can also be used for recursive queries. Here’s how you can find all employees under a specific manager:

WITH EmployeeHierarchy AS (<     SELECT EmployeeID, FirstName, LastName, ManagerID<     FROM Employees<     WHERE ManagerID IS NULL<     UNION ALL<     SELECT e.EmployeeID, e.FirstName, e.LastName, e.ManagerID<     FROM Employees e<     INNER JOIN EmployeeHierarchy eh ON e.ManagerID = eh.EmployeeID< )< SELECT * FROM EmployeeHierarchy;

Troubleshooting/FAQ

What if my CTE returns no results?

Double-check the conditions in your CTE. Ensure that the base query inside the CTE is constructed correctly and matches the expected data.

Can I use multiple CTEs?

Yes, you can define multiple CTEs by separating them with commas:

WITH CTE1 AS (...), CTE2 AS (...)< SELECT * FROM CTE1, CTE2;

Quick Checklist

• Prerequisites (tools/versions) are listed clearly.
• Setup steps are complete and reproducible.
• Include at least one runnable code example (SQL/Python/YAML).
• Explain why each step matters (not just how).
• Add Troubleshooting/FAQ for common errors.

2-Minute Case Study

Anita, 28, aims for ₹4 lakh emergency fund in 18 months. She picks a low-risk liquid/debt fund, sets a ₹22,000 SIP, and reviews once a quarter. For retirement, she chooses a Nifty 50 index fund with a 20-year SIP, increasing contributions 5% yearly.

FAQ

How much should I invest monthly?

Work backwards from goal and date; SIP = Goal ÷ Months (adjust for expected return).

Direct vs Regular plan?

Direct plans have lower expense ratios; over time that compounds to higher returns.

When should I sell?

Review annually. Rebalance if allocation drifts by >5–10% or when a goal is fully funded.

Related Reading

• Snowflake Basics: Setting Up Your Snowflake Account and Warehouse