Difference Between ALTER and UPDATE in SQL
Efficiently modifying and handling data is crucial in database management, and SQL offers a wide range of commands to facilitate these modifications. Two of the most fundamental commands are ALTER and UPDATE, which are often misunderstood despite serving distinct purposes crucial for effective database administration.
This article delves into the intricacies of ALTER and UPDATE commands in SQL, exploring their functionalities, differences, and appropriate use cases to enhance data integrity and efficiency.
The key difference between the 'ALTER' and 'UPDATE' commands in SQL lies in their primary purpose and the type of operation they perform on a database. ALTER command is used for changing a database table's structure (or schema), while the UPDATE command is used for modifying the data within the rows of an existing table.
Table of Content
- Difference Between ALTER and UPDATE Command in SQL: ALTER vs UPDATE
- What is the ALTER command in SQL?
- What is the UPDATE command in SQL?
- Key Difference Between ALTER and UPDATE Commands in SQL
What is the Difference Between ALTER and UPDATE in SQL?
| Parameter | ALTER Command | UPDATE Command |
|---|---|---|
| Purpose | Used to modify the structure of a database table. | Used to modify the data within a table. |
| Function | Changes the table schema by adding, deleting, or altering columns and constraints. | Updates the values in existing rows of the table. |
| Usage | 1. Adding or removing columns 2. Changing data types 3. Modifying constraints (e.g., adding a PRIMARY KEY) |
1. Changing values in specific rows and columns 2. Updating data based on conditions |
| Syntax Example | ALTER TABLE table_name ADD column_name datatype; ALTER TABLE table_name DROP COLUMN column_name; |
UPDATE table_name SET column1 = value1 WHERE condition; |
| Scope of Impact | Affects the table structure and layout. | Affects the data stored within the table. |
| Reversibility | Structural changes are harder to reverse and may require table recreation or restoration from a backup. | Data updates can be reversed if the original data is known or backed up. |
| Frequency of Use | Less frequent, usually during design changes or schema evolution. | More frequently, it is used in regular data manipulation and maintenance. |
| Impact on Data | Does not directly manipulate the data in the table (except when dropping columns). | Directly changes the data stored in the table's rows. |
| Execution Time | It can be time-consuming and resource-intensive for large tables, as it may require table restructure. | Generally faster than ALTER for small to moderate data changes, but can be slow for large-scale updates. |
| Considerations | Requires careful planning as it can affect database integrity and relationships. | Requires precision in specifying conditions to ensure correct data is updated. |
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What is the ALTER command in SQL?
ALTER command in SQL allows you to modify the structure of an existing table in a database without completely recreating it. This includes actions like:
- Adding or Removing Columns
- Changing data types of columns
- Renaming columns or tables
- Adding or removing constraints (rules for data integrity)
- Creating or deleting Indexes (for faster indexing)
Want to Learn More About How to Use the ALTER Command in SQL, check out our blog: SQL ALTER TABLE: ADD, DROP, MODIFY, RENAME
Syntax of ALTER Command in SQL
ALTER TABLE table_name ADD column_name datatype;
ALTER TABLE table_name DROP COLUMN column_name;
What is the UPDATE Command in SQL?
UPDATE command is used to modify the existing data within rows of a database table. It allows you to change the values of one or more columns for all rows that meet a certain condition. It's like a precision tool for making specific changes to your stored information.
Here's how it works:
- Specify the table: Clearly explain which table you want to update.
- Set the conditions: Define the exact rows that need changes using a WHERE clause.
- Assign new values: Indicate the columns you want to modify and provide the new values to be inserted.
Want to Learn More About How to Use the UPDATE Command in SQL, check out our blog: UPDATE Query in SQL.
Syntax of UPDATE Command in SQL
UPDATE table_nameSET column1 = value1, column2 = value2, ...WHERE condition;
Now, it's time for hands-on. So, let's take an example that will help you better understand the difference between ALTER and UPDATE commands in SQL.
Consider a Table: Employees (columns: Employee Name, Employee ID, Phone Number, and Address) that contains the record of 10 employees.
| Employee ID |
Employee Name |
Phone Number |
Address |
| 1001 |
Vaibhav |
+91-90-1234-1001 |
01, New Delhi |
| 1002 |
Vishal |
+91-90-1234-1002 |
02, New Delhi |
| 1003 |
Saumya |
+91-90-1234-1003 |
03, New Delhi |
| 1004 |
Arun |
+91-90-1234-1004 |
04, New Delhi |
| 1005 |
Anjum |
+91-90-1234-1005 |
05, New Delhi |
| 1006 |
Radhika |
+91-90-1234-1006 |
06, New Delhi |
| 1007 |
Harpreet |
+91-90-1234-1007 |
07, New Delhi |
| 1008 |
Sam |
+91-90-1234-1008 |
08, New Delhi |
| 1009 |
Tony |
+91-90-1234-1009 |
09, New Delhi |
| 1010 |
Victoria |
+91-90-1234-1010 |
10, New Delhi |
Now, the question is to add a new column named 'Department' to the Employee table and then update this column with the department names for each employee.
Step 1: Use the ALTER command to Add a New Column
ALTER TABLE Employees ADD Department VARCHAR(255);
Output
| Employee ID |
Employee Name |
Phone Number |
Address |
Department |
| 1001 |
Vaibhav |
+91-90-1234-1001 |
01, New Delhi |
|
| 1002 |
Vishal |
+91-90-1234-1002 |
02, New Delhi |
|
| 1003 |
Saumya |
+91-90-1234-1003 |
03, New Delhi |
|
| 1004 |
Arun |
+91-90-1234-1004 |
04, New Delhi |
|
| 1005 |
Anjum |
+91-90-1234-1005 |
05, New Delhi |
|
| 1006 |
Radhika |
+91-90-1234-1006 |
06, New Delhi |
|
| 1007 |
Harpreet |
+91-90-1234-1007 |
07, New Delhi |
|
| 1008 |
Sam |
+91-90-1234-1008 |
08, New Delhi |
|
| 1009 |
Tony |
+91-90-1234-1009 |
09, New Delhi |
|
| 1010 |
Victoria |
+91-90-1234-1010 |
10, New Delhi |
Step 2: Use the UPDATE command to Populate the New Column
Now, we'll update the Department column for each employee. Let's say employees with IDs from 1001 to 1005 belong to 'Sales' and the rest to 'Marketing'.
SQL Command for Sales:
UPDATE Employees SET Department = 'Sales' WHERE EmployeeID BETWEEN 1001 AND 1005;
Must Check: Between Operator in SQL
SQL Command for Marketing:
UPDATE Employees SET Department = 'Marketing' WHERE EmployeeID > 1005;
Must Check: SQL Comparison and Arithmetic Operator
Output
| Employee ID |
Employee Name |
Phone Number |
Address |
Department |
| 1001 |
Vaibhav |
+91-90-1234-1001 |
01, New Delhi |
Sales |
| 1002 |
Vishal |
+91-90-1234-1002 |
02, New Delhi |
Sales |
| 1003 |
Saumya |
+91-90-1234-1003 |
03, New Delhi |
Sales |
| 1004 |
Arun |
+91-90-1234-1004 |
04, New Delhi |
Sales |
| 1005 |
Anjum |
+91-90-1234-1005 |
05, New Delhi |
Sales |
| 1006 |
Radhika |
+91-90-1234-1006 |
06, New Delhi |
Marketing |
| 1007 |
Harpreet |
+91-90-1234-1007 |
07, New Delhi |
Marketing |
| 1008 |
Sam |
+91-90-1234-1008 |
08, New Delhi |
Marketing |
| 1009 |
Tony |
+91-90-1234-1009 |
09, New Delhi |
Marketing |
| 1010 |
Victoria |
+91-90-1234-1010 |
10, New Delhi |
Marketing |
From the above example, we depict that the ALTER command was used to add a new column (i.e., it is used to change the structure or schema of the existing table). Then the UPDATE command was used to fill the entries in the new column (i.e., the update command is used to change or modify the existing table's entries without changing the table's structure or schema).
Key Differences and Similarities Between ALTER and UPDATE Command in SQL
- ALTER command is primarily used for changing table architect, such as adding, deleting, or modifying columns in an existing table. In contrast, the UPDATE command is used to update the value in the existing rows of the table.
- ALTER command can have a long-term impact on the database schema, while UPDATE affects the current data.
- Mistakes in UPDATE can be reversed if the old data is known, but ALTER changes can be harder to reverse without a backup.
- UPDATE commands are generally faster than ALTER for minor to moderate changes but can be slow for large-scale updates.
- Executing either ALTER or UPDATE requires certain user permissions on the database and the specific tables being modified. This is part of database security and access control.
- Depending on the database setup, ALTER and UPDATE can trigger other processes like constraint checks, cascades, or trigger executions.
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FAQs on the Difference Between ALTER and UPDATE Command in SQL
What is the ALTER command used for in SQL?
The ALTER command is used to modify the structure of an existing table in a database. It can add or remove columns, change data types, and modify constraints.
How do I add a new column to a table using the ALTER command?
To add a new column, use: ALTER TABLE table_name ADD column_name datatype;. For example, ALTER TABLE Employees ADD Email VARCHAR(255);.
Can I use the ALTER command to delete a column?
Yes, use: ALTER TABLE table_name DROP COLUMN column_name;. For example, ALTER TABLE Employees DROP COLUMN Email;.
What does the UPDATE command do in SQL?
The UPDATE command is used to modify existing data within a table. It can change the values in one or more columns for all rows that meet a certain condition
How can I update data in specific rows using the UPDATE command?
Use the UPDATE command with a WHERE clause to specify the condition: UPDATE table_name SET column1 = value1 WHERE condition;. For example, UPDATE Employees SET Phone_Number = '123-456-7890' WHERE EmployeeID = 1001;.
Is it possible to update multiple columns at once using UPDATE?
Yes, separate the columns with commas: UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;.
Can ALTER command be used to change a column's name?
Yes, the syntax depends on the SQL database system. In many systems, it's:ALTER TABLE table_name RENAME COLUMN old_name TO new_name;.
How do I add a primary key to a table using ALTER?
Use: ALTER TABLE table_name ADD PRIMARY KEY (column_name);. Ensure the column has unique values and no NULLs.
What happens if the UPDATE command is run without a WHERE clause?
The update will apply to all rows in the table, which can lead to unintended data changes. Always use the WHERE clause to specify which rows should be updated.
Vikram has a Postgraduate degree in Applied Mathematics, with a keen interest in Data Science and Machine Learning. He has experience of 2+ years in content creation in Mathematics, Statistics, Data Science, and Mac... Read Full Bio
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