SQL Tables

What is a SQL Table?

A table in SQL is a collection of data organized in rows and columns. Each row represents a record, and each column represents an attribute or field of the record. SQL tables are used to store structured data in relational databases, where each table represents a specific entity (e.g., customers, orders, products).

Tables are the fundamental storage objects in a relational database. A well-designed table helps maintain data integrity and optimizes database performance.

Creating a Table in SQL

You can create a table in SQL using the CREATE TABLE statement. Here’s an example:

CREATE TABLE Customers (
    CustomerID int,
    FirstName varchar(50),
    LastName varchar(50),
    Email varchar(100),
    Phone varchar(20),
    PRIMARY KEY (CustomerID)
);

This statement creates a table called Customers with columns for CustomerID, FirstName, LastName, Email, and Phone. The CustomerID is defined as the primary key, which uniquely identifies each customer.

SQL Table Constraints

Constraints in SQL are rules applied to columns in a table to ensure data accuracy and integrity. Here are some common constraints used in SQL tables:

• PRIMARY KEY: Ensures each record in a table is unique.
• FOREIGN KEY: Links two tables together, establishing a relationship between them.
• NOT NULL: Ensures that a column cannot have a NULL value.
• UNIQUE: Ensures all values in a column are different.
• CHECK: Ensures that all values in a column meet a specific condition.

Inserting Data into a Table

Once a table is created, you can insert data into it using the INSERT INTO statement. Here’s an example:

INSERT INTO Customers (CustomerID, FirstName, LastName, Email, Phone)
VALUES (1, 'Arshiya', 'Naaz', 'Arshiya.Naaz@example.com', '555-1234');

This statement adds a new record to the Customers table with the values for CustomerID, FirstName, LastName, Email, and Phone.

Querying Data from a Table

To retrieve data from a table, you can use the SELECT statement. Here’s an example that retrieves all the data from the Customers table:

SELECT * FROM Customers;

This query selects all columns from the Customers table. You can also specify which columns to retrieve:

SELECT FirstName, LastName, Email FROM Customers;

Updating Data in a Table

To update existing data in a table, you can use the UPDATE statement. Here’s an example:

UPDATE Customers
SET Email = 'MohammadMaaz@gmail.com'
WHERE CustomerID = 1;

This statement updates the email address for the customer with a CustomerID of 1.

Deleting Data from a Table

To delete data from a table, use the DELETE statement. Here’s an example:

DELETE FROM Customers
WHERE CustomerID = 1;

This statement deletes the record where the CustomerID is 1 from the Customers table.