Updating Data

Introduction

The ability to update data is a crucial feature in any SQL-based database management system. SQL provides the UPDATE statement, which allows you to modify existing records within a table. Whether you want to change the values of a single row, a subset of rows, or every row in a table, the UPDATE command offers flexibility and power.

This section will explore how to use the UPDATE statement in SQL, with special attention to MS SQL Server. We will cover basic update operations, conditional updates, updating multiple columns, and practical examples using customer management scenarios.

Update Statement

The UPDATE statement in SQL is used to update data in a table. With this command, you can change an existing record in a table according to a given condition.

Syntax

UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;

• SET: Specifies the column(s) to be updated and their new values.
• WHERE: Defines the condition(s) that determine which rows will be updated. If you omit the WHERE clause, all rows in the table will be updated, which might not be desirable.

Example

UPDATE Customers
SET first_name = 'Alex'
WHERE customer_id = 1525;

Things to keep in mind while using UPDATE Statement

• Always use the WHERE clause to specify which rows should be updated. Without it, all rows will be updated!
• Before updating values, confirm which rows are being modified by SELECT statement.

Conclusion

A UPDATE statement can be used to update data, but be careful with what you are updating and always use the WHERE clause.

Asked in Interview

• What happens if you omit the WHERE clause in an UPDATE statement?
• What are the potential risks of using an UPDATE statement?
• How do you update multiple columns in a single SQL UPDATE statement?
• Can you use the ORDER BY clause in an UPDATE statement?
• What are the best practices for updating large datasets in SQL?