What Happens to Data When You Alter the Table Structure?


Scott Campbell

What Happens to Data When You Alter the Table Structure?

When you alter the structure of a table in a database, it is important to understand how this affects the existing data. In this article, we will explore what happens to the data when you make changes to the table structure.

Altering Column Data Type

One common alteration is changing the data type of a column. For example, you may want to change a column from an integer data type to a string data type. When you perform this alteration, the database will attempt to convert the existing data in that column to fit the new data type.

However, it is crucial to note that this conversion may lead to data loss or truncation. For instance, if the original column had values like “123” and “456”, changing it to a string data type would preserve those values. On the other hand, if the original column had values like “123” and “4567”, changing it to an integer data type would result in truncation of the last digit.

Adding or Removing Columns

Another alteration commonly performed on tables is adding or removing columns. Adding a new column does not affect existing data. The newly added column is empty and can be populated later with appropriate values.

On the other hand, removing a column permanently deletes all data stored in that column. Therefore, it is crucial to ensure that you have backed up any essential information before removing columns from your table structure.

Modifying Constraints

Constraints are rules that define certain conditions for your table’s columns. Common constraints include primary keys, foreign keys, unique constraints, and check constraints.

If you alter constraints on your table structure, such as modifying or removing a primary key, the database will validate the existing data against the new constraints. If any data violates the new constraints, you may receive errors or be required to modify or remove the conflicting data before making the alterations.

Renaming a Table

Renaming a table does not affect the data within it. The table’s name is merely a reference, and changing it will not alter any data stored in that table.


When altering the structure of a table in your database, it is essential to understand how these changes impact your existing data. Changing column data types can result in data loss or truncation, while adding or removing columns can have implications on stored information.

Modifying constraints may require you to validate and modify existing data to adhere to new rules. However, renaming a table does not affect any of its stored data.

By understanding what happens to data when you alter the table structure, you can make informed decisions and ensure the integrity of your database.

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