SQL Table Queries

Table Queries

In SQL, data is stored inside tables. A table is like an Excel sheet where:

Before we can store any data, we must first create a table.

Later, we may need to change its structure or delete it completely.

For this purpose, SQL provides Table Queries.

CREATE TABLE - Creating a Table

The CREATE TABLE command is used to create a new table inside a database.

Syntax

CREATE TABLE table_name (
    column_name data_type,
    column_name data_type
);

Example

CREATE TABLE Students (
    StudentID INT,
    Name VARCHAR(50),
    Age INT
);

After this command, an empty table is created ready to store data.

ALTER TABLE - Changing Table Structure

Sometimes, after creating a table, you realize that:

In such cases, we use ALTER TABLE.

Add a New Column

ALTER TABLE Students
ADD City VARCHAR(30);

Adds a new column City to the table.

Change Column Data Type

ALTER TABLE Students
MODIFY Age VARCHAR(3);

Changes the type of Age column.

Remove a Coloumn

ALTER TABLE Students
DROP COLUMN City;

Deletes the column from the table.

Always be careful altering tables may affect existing data.

DROP TABLE - Deleting a Table

The DROP TABLE command removes the table permanently.

Syntax

DROP TABLE table_name;

Example

DROP TABLE Students;

This deletes:

There is no undo, so use carefully.

Two Minute Drill