# Constraints in SQL

## 🔐 What Are Constraints in SQL?

**Constraints** are **rules** applied to table columns to **control the type of data** that can go into a table. They **protect your data** and make sure it stays accurate and consistent.

---

## ✅ Types of SQL Constraints

| Constraint | Description |
| --- | --- |
| `NOT NULL` | Column cannot be empty |
| `UNIQUE` | All values in the column must be different |
| `PRIMARY KEY` | Uniquely identifies each row (combines `NOT NULL` + `UNIQUE`) |
| `FOREIGN KEY` | Links to a column in another table |
| `CHECK` | Ensures values meet a condition |
| `DEFAULT` | Sets a default value if none is provided |

---

### 1️⃣ `NOT NULL`

Means the column **must have a value**.

```sql
CREATE TABLE Students (
  ID INT NOT NULL,
  Name VARCHAR(50) NOT NULL
);
```

👉 You cannot insert a student without `ID` or `Name`.

---

### 2️⃣ `UNIQUE`

Makes sure all values in that column are **different**.

```sql
CREATE TABLE Users (
  Email VARCHAR(100) UNIQUE
);
```

👉 No two users can have the same email.

---

### 3️⃣ `PRIMARY KEY`

* Combines `NOT NULL` and `UNIQUE`.
    
* Only one **primary key per table**.
    
* Used to **identify rows** uniquely.
    

```sql
CREATE TABLE Employees (
  EmpID INT PRIMARY KEY,
  Name VARCHAR(50)
);
```

👉 Each `EmpID` must be unique and not null.

---

### 4️⃣ `FOREIGN KEY`

Used to **link one table to another**.

```sql
CREATE TABLE Orders (
  OrderID INT PRIMARY KEY,
  CustomerID INT,
  FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);
```

👉 `CustomerID` in `Orders` must match an existing `CustomerID` in `Customers` table.

---

### 5️⃣ `CHECK`

Makes sure the value follows a **certain rule**.

```sql
CREATE TABLE Products (
  Price INT CHECK (Price > 0)
);
```

👉 Price must always be greater than 0.

---

### 6️⃣ `DEFAULT`

Sets a value **automatically** if not provided.

```sql
CREATE TABLE Accounts (
  Balance INT DEFAULT 0
);
```

👉 If no balance is given, it will be 0 by default.

---

## ✅ Summary Table:

| Constraint | Can Be Null? | Must Be Unique? | Extra |
| --- | --- | --- | --- |
| NOT NULL | ❌ | 🚫 | Field required |
| UNIQUE | ✅ | ✅ | No duplicates |
| PRIMARY KEY | ❌ | ✅ | One per table |
| FOREIGN KEY | ✅ (depends) | 🚫 | Must match other table |
| CHECK | ✅ | 🚫 | Custom rules |
| DEFAULT | ✅ | 🚫 | Auto value if empty |

## 🎓 Example Table: `Students`

### 🎯 Goal:

We want to create a table that:

* Has a unique ID for every student
    
* Requires name and age
    
* Doesn't allow duplicate roll numbers
    
* Ensures age is at least 5
    
* Assigns a default country if not given
    
* Links to another table for class info
    

---

### ✅ Step-by-step SQL Code with Constraints:

```sql
CREATE TABLE Students (
  StudentID INT PRIMARY KEY,                -- Each student must have a unique ID (not null + unique)
  RollNumber INT UNIQUE,                    -- No two students can have same roll number
  Name VARCHAR(50) NOT NULL,                -- Name is required
  Age INT CHECK (Age >= 5),                 -- Age must be 5 or more
  Country VARCHAR(50) DEFAULT 'India',      -- If country not given, default is India
  ClassID INT,                              -- ClassID will link to another table
  FOREIGN KEY (ClassID) REFERENCES Classes(ClassID)  -- Must match a ClassID in Classes table
);
```

---

### 🔍 Now Let's Understand Each Constraint:

| Part | Constraint Used | Meaning |
| --- | --- | --- |
| `StudentID INT PRIMARY KEY` | `PRIMARY KEY` | Makes `StudentID` unique and required (NOT NULL + UNIQUE) |
| `RollNumber INT UNIQUE` | `UNIQUE` | Ensures every student has a different roll number |
| `Name VARCHAR(50) NOT NULL` | `NOT NULL` | Student must have a name |
| `Age INT CHECK (Age >= 5)` | `CHECK` | Age must be at least 5 |
| `Country VARCHAR(50) DEFAULT 'India'` | `DEFAULT` | If no country is entered, it will be set as 'India' |
| `ClassID INT` + `FOREIGN KEY` | `FOREIGN KEY` | ClassID must exist in another table called `Classes` |

---

### 📄 Related Table: `Classes`

```sql
CREATE TABLE Classes (
  ClassID INT PRIMARY KEY,
  ClassName VARCHAR(30)
);
```