SQL ALTER TABLE Assertion

Introduction

ALTER command is likely one of the most necessary SQL features to know for database manipulation and administration. This Knowledge Definition Language (DDL) command can alter present tables by including, eradicating, or updating columns, constraints, and indexes. We’ll take 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 alter the info kind of an present column to match new knowledge wants.

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Overview

• Study the aim and performance of the SQL ALTER command.
• Perceive how and the place to make use of SQL’s ALTER TABLE statements.
• Discover varied situations and examples demonstrating using this SQL perform.

Purposes of ALTER Command

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

Pattern Desk

Right here’s a pattern desk on which we will likely be testing out varied 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 change an present column, use the MODIFY or ALTER COLUMN clause.

ALTER TABLE workers
MODIFY COLUMN birth_date DATETIME;

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

Dropping a Column

To take away an present 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 staff desk.

Including Constraints

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

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

This can add a major 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 can take away the first key constraint (pk_employee_id) from the Staff desk.

Renaming a Desk

The ALTER command may also rename a desk.

ALTER TABLE workers
RENAME TO employees;

This can 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 flexibleness required for adaptable database design and administration. This command makes it straightforward to replace database constructions as wants change.

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