Data Constraints

Data constraints in SQL are used to specify rules for the data in a table. Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table.

Types of SQL Data Constraints

NOT NULL Constraint: Ensures that a column cannot have a NULL value.

For Example:

CREATE TABLE Students (
    ID int NOT NULL,
    Name varchar(255) NOT NULL,
    Age int
);

UNIQUE Constraint: Ensures that all values in a column are different.

For Example:

CREATE TABLE Students (
    ID int NOT NULL UNIQUE,
    Name varchar(255) NOT NULL,
    Age int
);

PRIMARY KEY Constraint: Uniquely identifies each record in a database table. Primary keys must contain UNIQUE values. Exactly the same as the UNIQUE constraint but there can be many unique constraints in a table, but only one PRIMARY KEY constraint per table.

For Example:
```
CREATE TABLE Students (
    ID int NOT NULL,
    Name varchar(255) NOT NULL,
    Age int,
    PRIMARY KEY (ID)
);
```
FOREIGN KEY Constraint: Prevents actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.

For Example:
```
CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    ID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (ID) REFERENCES Students(ID)
);
```

CHECK Constraint: The CHECK constraint ensures that all values in a column satisfies certain conditions.

For Example:

CREATE TABLE Students (
    ID int NOT NULL,
    Name varchar(255) NOT NULL,
    Age int,
    CHECK (Age>=18)
);

DEFAULT Constraint: Provides a default value for a column when none is specified.

For Example:

CREATE TABLE Students (
    ID int NOT NULL,
    Name varchar(255) NOT NULL,
    Age int,
    City varchar(255) DEFAULT 'Unknown'
);

INDEX Constraint: Used to create and retrieve data from the database very quickly.

For Example:
```
CREATE INDEX idx_name 
ON Students (Name);
```
Note: Indexes are not a part of the SQL standard and are not supported by all databases.

Data Constraints

Types of SQL Data Constraints​

Types of SQL Data Constraints