SQL ALTER TABLE Assertion

Introduction

ALTER command is without doubt one of the most vital SQL features to know for database manipulation and administration. This Information Definition Language (DDL) command can alter current tables by including, eradicating, or updating columns, constraints, and indexes. We’ll have a look at alternative ways to make use of the ALTER command to focus on its flexibility. As well as, ALTER TABLE statements in SQL could also be used to vary the info kind of an current column to match new knowledge wants.

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Overview

• Be taught the aim and performance of the SQL ALTER command.
• Perceive how and the place to make use of SQL’s ALTER TABLE statements.
• Discover numerous eventualities and examples demonstrating the usage of this SQL operate.

Functions of ALTER Command

The ALTER command in SQL can be utilized for a number of functions. Listed below are some widespread purposes:

Pattern Desk

Right here’s a pattern desk on which we will probably be testing out numerous ALTER TABLE statements.

Including a New Column

So as to add a brand new column in our Worker desk, you should use the ADD COLUMN assertion.

ALTER TABLE workers
ADD COLUMN birth_date DATE;
SELECT emp_id, birth_date from Staff WHERE emp_id BETWEEN 1 AND 5;

This provides a brand new column named `birth_date` of kind DATE within the desk, however the values are empty as we simply added a brand new column in Staff.

Modifying an Current Column

To switch an current column, use the MODIFY or ALTER COLUMN clause.

ALTER TABLE workers
MODIFY COLUMN birth_date DATETIME;

The `birth_date` column within the `workers` desk adjustments from DATE to DATETIME kind. Equally we are able to change different knowledge varieties as properly for different columns.

Dropping a Column

To take away an current column out of your desk, use the DROP COLUMN in SQL.

ALTER TABLE workers
DROP COLUMN birth_date;

This SQL question removes the birth_date column from the workers desk.

Including Constraints

The ALTER command’s makes use of lengthen to including constraints to a desk like including a main key, international key, or distinctive constraint.

ALTER TABLE workers
ADD CONSTRAINT pk_employee_id PRIMARY KEY (emp_id);

This may add a main key constraint named `pk_employee_id` on the `emp_id` column of the `workers` desk.

Eradicating Constraints

We use the DROP CONSTRAINT to take away a constraint from the desk.

ALTER TABLE workers
DROP CONSTRAINT pk_employee_id;

This may take away the first key constraint (pk_employee_id) from the Staff desk.

Renaming a Desk

The ALTER command may rename a desk.

ALTER TABLE workers
RENAME TO employees;

This may rename the `workers` desk to `employees`.

Conclusion

DDL’s ALTER command may be very efficient for managing and altering database desk constructions. Customers can add, change, or delete columns and constraints, offering the pliability required for adaptable database design and administration. This command makes it simple to replace database constructions as wants change.

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