SQL ALTER TABLE Assertion

Introduction

ALTER command is among the most necessary 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 take a look at other ways to make use of the ALTER command to spotlight its flexibility. As well as, ALTER TABLE statements in SQL could also be used to alter the information sort of an current column to match new information 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 varied eventualities and examples demonstrating the usage of this SQL perform.

Purposes of ALTER Command

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

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 need to 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 sort DATE within the desk, however the values are empty as we simply added a brand new column in Staff.

Modifying an Present 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 modifications from DATE to DATETIME sort. Equally we will change different information sorts as effectively 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 major key, overseas key, or distinctive constraint.

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

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

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