A Brief Guide to Working With ‘ALTER’ Command in SQL-Know the Ins and Outs!

Structured Query Language (SQL) is necessary for most, if not all, industries worldwide. Starting from IT sector to finance and even healthcare, SQL makes its way through every single domain to enhance the efficiency of its technical aspects.

SQL is essential to Database Management Systems (DBMS) because it offers a standardised interface for interacting with relational databases. SQL allows users to conduct various actions, including obtaining data, updating the structure of databases, and controlling data access. Knowledge of SQL is highly regarded and sought-after by companies due to the language’s prominence in the business.

The ALTER command in SQL is a command used to modify the structure of a database object, such as a table, view, index, stored procedure, or function. Every part of a database object can be modified using the ALTER command, including the number of columns in a table, the data type of a column, and even the description of a stored procedure.

It’s a robust command with far-reaching potential for altering a database’s structure, but it should be handled with care because of the gravity of the potential implications of doing so poorly. When issuing an ALTER command, it’s crucial to know exactly what changes will be made and to have sufficient backups and safeguards to prevent data loss or corruption.

Let us delve into ‘working with the ALTER command in SQL’ so that you know everything whilst implementing it in a production environment. 

Types of ALTER commands

There are different types of ALTER commands used in SQL, each having its own significance. The various types are as follows:


The SQL ALTER TABLE command is used to modify the structure of a table in a SQL database. Modifying a table’s columns might include adding new ones, altering the columns’ data type or length, or removing them altogether. SQL ALTER TABLE‘s syntax changes based on the nature of the modification being done. However, some frequent instances are as follows:

  • Adding a new column to a table:

ALTER TABLE table_name ADD new_column_name data_type

  • Changing the data type of an existing column:
    ALTER TABLE table_name ALTER COLUMN name_of_the_column new_data_type
  • Dropping a column from a table:
    ALTER TABLE table_name DROP COLUMN name_of_the_column


The ALTER VIEW command is used to modify the definition of a view in a SQL database. A view is a virtual table that may be used to access information from several physical tables using a single SQL statement. Using the ALTER VIEW command, you may alter the SELECT statement that specifies the view to include new or different columns, apply new or different filters, or even create new groups. Below is the format for the ALTER VIEW command:

  • ALTER VIEW view_name AS new_select_statement

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The ALTER INDEX command is used to modify the structure of an index in a SQL database. Query speed can be enhanced by creating an ‘index’, which is a data structure that allows for a quick lookup of rows based on the values in one or more columns. You can use the ALTER INDEX command to rearrange the columns in an existing index or change their data type or order. The format of the ALTER INDEX command is as follows:

  • ALTER INDEX name_of_index ON table_name 

You can use the ALTER INDEX command and then carry out other queries such as RENAME, SET, RESET, etc.

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The ALTER PROCEDURE/FUNCTION command is used to modify the definition of a stored procedure or function in a SQL database. A stored procedure or function is a collection of SQL statements that may be run together to accomplish a specific task.

Using the ALTER PROCEDURE/FUNCTION command, you can alter the stored procedure or function internal code to add or delete SQL statements, alter parameter lists, or change the return type. Below is the basic syntax for the ALTER PROCEDURE/FUNCTION command:

  • ALTER PROCEDURE procedure_name [parameter_list] AS new_procedure_body
  • ALTER FUNCTION function_name [parameter_list] RETURNS return_data_type AS new_function_body

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Must Read: Rename Column Name in SQL

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Difference between ALTER and UPDATE

ALTER is one of the DDL(Data Definition Language) commands, the other ones being CREATE, DROP, TRUNCATE and RENAME. Structured Query Language’s DDL commands allow users to construct and alter the database’s underlying data model. Most SQL learners do get confused between ALTER and UPDATE.

Let us compare the ALTER command to the UPDATE command below, so you have a better understanding of what to use on your database:

1. ALTER is a DDL(Data Definition Language) command. UPDATE is a DML(Data Manipulation Language) command.
2. The database’s relations’ (tables’) attributes may be modified, added to, or removed with the use of the ALTER command. The UPDATE command can be used to modify pre-existing database entries in a table.
3. By default, the ALTER command sets all tuple values to NULL. UPDATE assigns the values to the tuple that are provided along with the command.
4. Operates on a structure level Operates on a data level

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The ALTER command is an important tool in database management systems. The proper use of this command is essential for maintaining data integrity, which is why it’s important for database administrators to understand best practices for using the ALTER command.

These include backing up data before making any changes, avoiding modifications to primary keys or indexed columns, keeping track of all changes made using comments, and testing the new structure before implementing it. By adhering to these best practices, database managers assure the security and efficacy of their databases.

SQL is widely used in the domain of data science as it provides a simple and efficient way for data scientists to retrieve, transform, analyse, visualise, and integrate data from a wide variety of sources.

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Can I use the ALTER command to rename a table?

Yes, you can use the ALTER command to rename an existing table using the following SQL statement: ALTER TABLE old_table_name RENAME TO new_table_name For example, to rename a table named employees to staff, you can use the following command: ALTER TABLE employees RENAME TO staff

Can I use the ALTER command to change the data type of an existing column without losing data?

It depends on the particular data type you want to modify. If the new data type cannot contain the old data, changing the data type of a column may occasionally result in data loss or corruption.

Can I use the ALTER command to add a primary key constraint to a table?

Yes, you can use the ALTER command to add a primary key constraint to an existing table. Let’s say that you want to add a primary key constraint to a table named users on the id column. Below is the syntax to implement the same: ALTER TABLE users ADD PRIMARY KEY (id)

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