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ALTER vs. UPDATE: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on February 9, 2024
ALTER is used to change the structure of a database table (like adding a column), while UPDATE modifies the data within the table (like changing a value).

Key Differences

ALTER is a Data Definition Language (DDL) command in SQL used to modify the structure of a database table, such as adding or deleting columns, changing data types, or renaming tables. UPDATE, on the other hand, is a Data Manipulation Language (DML) command used to modify existing data within the rows of a table. It can change the values of specified columns in one or multiple rows.
The ALTER command is crucial when the structure of a database table needs to be altered without affecting the data contained within it. It allows for changes that can impact how data is organized and stored. The UPDATE command, in contrast, is used for modifying the data within the database without altering the table's structure. It's used when the existing data in the database needs to be revised or corrected.
When using ALTER, the changes made to the table structure can have significant implications on the database design and integrity. It's a command that needs careful planning and consideration. UPDATE is used for regular operations within the database, such as changing a customer's address or updating a product price. It's a more frequently used command compared to ALTER as data often requires updates based on business operations.
ALTER can be used to add new constraints to a table, like adding a foreign key or setting a default value for a column. This changes how data will be handled in the future but doesn't modify existing data. UPDATE, however, is used to change existing records within a table, like updating an employee's salary. It directly affects the data currently stored in the database.
The use of ALTER requires understanding the architecture and design of the database, as it can significantly alter how the database functions. It's often used less frequently than UPDATE, as structural changes are not as common as data updates. UPDATE is part of the routine data management operations, ensuring the data within the database remains relevant and accurate.

Comparison Chart

Type of Command

Data Definition Language (DDL)
Data Manipulation Language (DML)


Modifies the structure of a database table
Modifies the data within the database table

Examples of Use

Adding or deleting columns, changing data types
Changing the values of existing records


Affects table design and architecture
Affects the content within the existing records

Frequency of Use

Less frequent, structural changes
More frequent, data revision and correction

ALTER and UPDATE Definitions


ALTER affects how data is stored and organized.
ALTER TABLE Products RENAME TO Inventory; changes the name of the Products table to Inventory.


UPDATE ensures data accuracy and relevance.
UPDATE Users SET LastLogin = CURRENT_TIMESTAMP WHERE UserID = 'john_doe'; updates the last login time for a user.


ALTER changes the structure of a database table.
The command ALTER TABLE Students ADD COLUMN Birthdate DATE; adds a new column to the Students table.


UPDATE modifies existing data in a database table.
UPDATE Customers SET Address = '123 New Lane' WHERE CustomerID = 1; changes the address of a specific customer.


ALTER adds or removes constraints in a table.
ALTER TABLE Orders ADD CONSTRAINT FK_Customer FOREIGN KEY (CustomerID) REFERENCES Customers(ID); adds a foreign key constraint.


UPDATE is used for revising table records.
UPDATE Employees SET Salary = Salary * 1.05; gives a 5% salary raise to all employees.


ALTER modifies table architecture in a database.
ALTER TABLE Orders DROP COLUMN Discount; removes a column from the Orders table.


UPDATE changes values in existing database rows.
UPDATE Products SET Price = Price - 10 WHERE Category = 'Electronics'; reduces the price of all electronics.


ALTER is used to redefine a table's schema.
ALTER TABLE Employees ALTER COLUMN Salary DECIMAL(10,2); changes the data type of the Salary column.


UPDATE is part of regular data management.
UPDATE Orders SET Status = 'Shipped' WHERE OrderID = 500; updates the status of an order.


To change or make different; modify
Altered my will.


To alter so as to be up to date
Update a textbook.
Update the files.


Can ALTER add a new column to a table?

Yes, ALTER can add new columns to an existing table.

What can you not do with an ALTER command?

ALTER cannot modify existing data within the table.

Can ALTER change a column's data type?

Yes, it can change the data type of a column.

Does UPDATE change the table structure?

No, UPDATE only modifies the data within the table, not its structure.

What is the purpose of the UPDATE command?

To change or correct existing data within database table rows.

Is ALTER a DML command?

No, it's a Data Definition Language (DDL) command.

Does ALTER work on multiple tables at once?

No, it typically works on one table at a time.

Can UPDATE affect multiple rows at once?

Yes, UPDATE can modify multiple rows based on the given criteria.

Is UPDATE used for adding new rows?

No, UPDATE is only for modifying existing rows; INSERT is used for adding new rows.

Does UPDATE need a WHERE clause?

While not mandatory, a WHERE clause is recommended to target specific rows.

What happens if you omit the WHERE clause in an UPDATE statement?

It can potentially update all rows in the table, which might be unintended.

What does ALTER do in SQL?

Modifies the structure or design of a database table.

Does the UPDATE command affect database performance?

Yes, especially if it involves large data sets or lacks proper indexing.

Is the UPDATE command reversible?

Yes, if used within a transaction that can be rolled back.

Can ALTER command be rolled back?

No, as a DDL command, it's auto-committed and cannot be rolled back.

What's a common use of UPDATE in databases?

Updating user information, product prices, or order statuses.

Is ALTER reversible in any situation?

Generally, no, once executed, the changes are permanent.

Are ALTER commands frequent in database management?

Less frequent compared to UPDATE, as structural changes are less common.

Can UPDATE be used with JOIN?

Yes, UPDATE can be used with JOIN to update rows based on other tables.

Can ALTER remove a column?

Yes, it can permanently remove a column from a table.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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