SQL Server Data Types and Constraints

Welcome to Day 1 of the 30-Day SQL Server Revision Challenge! 🎉 This series is perfect for those looking to brush up on SQL Server fundamentals in a structured, quick 30 minutes and fun way.

Today, we’re starting with the building blocks of any database: Data Types and Constraints. Let’s dive in. 👇

🧱 SQL Server Data Types

SQL Server provides various data types to define the kind of data each column can store. Here are the most commonly used:

📊 Numeric Data Types

INT: Integer values (whole numbers)
FLOAT: Approximate numeric with floating decimal
DECIMAL(p,s): Precise numeric with scale (e.g., DECIMAL(5,2) = 999.99)

🔤 Character Data Types

VARCHAR(n): Variable-length non-Unicode string (up to 8,000 chars)
NVARCHAR(n): Variable-length Unicode string (international chars)

📆 Date and Time Types

DATETIME: Stores both date and time
DATE: Date only
TIME: Time only

⚙️ Misc

BIT: Boolean values (0 = false, 1 = true)

🚦 SQL Constraints

Constraints are rules applied to table columns to enforce data integrity.

1. PRIMARY KEY 🔑

Ensures each record is unique and not NULL.

CREATE TABLE Employees (
   EmpID INT PRIMARY KEY,
   Name VARCHAR(100)
);

2. FOREIGN KEY 🔗

Links one table’s column to another’s primary key.

CREATE TABLE Orders (
   OrderID INT PRIMARY KEY,
   EmpID INT FOREIGN KEY REFERENCES Employees(EmpID)
);

3. CHECK ✅

Enforces domain-level constraints.

Age INT CHECK (Age >= 18)

4. DEFAULT 🎁

Assigns default values.

IsActive BIT DEFAULT 1

5. NOT NULL ❌

Prevents NULLs in a column.

Email VARCHAR(100) NOT NULL

🧠 Pro Tip:

If you’re ever unsure about what data type to use, start small and scale as needed. Use NVARCHAR for multilingual support and DECIMAL when precision matters (e.g., for money).

📌 Summary Cheat Sheet

Category	Examples
Numeric	INT, FLOAT, DECIMAL
String	VARCHAR, NVARCHAR
Date	DATETIME, DATE, TIME
Boolean	BIT

Constraint	Purpose
PRIMARY KEY	Uniqueness
FOREIGN KEY	Relational Integrity
CHECK	Domain Rules
DEFAULT	Auto Value
NOT NULL	Mandatory Field

💡 Day 1 Challenge:
Create a table named Students with the following:

StudentID (Primary Key)
Name (Not Null)
Age (Check: 5 to 100)
EnrollmentDate (Default: Current Date)

Share your solution in the comments or post it using the tag #30DaySQLChallenge. Let’s build this together! 🚀

Stay tuned for Day 2: SELECT Essentials tomorrow! 👀

SQL Server Kickoff – What Data Fits Where?

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