SQL UPDATE Statement

The UPDATE statement in SQL is used to modify existing data within a table. It lets you change values in one or more columns for selected rows.

UPDATE: Definition and Usage

UPDATE is a core command for maintaining database accuracy. It's crucial to always include a WHERE clause to specify exactly which rows to modify; otherwise, the entire table will be updated, potentially leading to significant data loss. This is because the `UPDATE` statement will change every row in the table if no `WHERE` clause is specified.

Syntax

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

Example Database

Here's a sample 'Customers' table to illustrate:

Examples

Updating a Single Record

This updates the contact name and city for CustomerID 1.

UPDATE Customers
SET ContactName = 'Alfred Schmidt', City = 'Frankfurt'
WHERE CustomerID = 1;
      

Updating Multiple Records

This updates the 'ContactName' for all Mexican customers.

UPDATE Customers
SET ContactName = 'Juan'
WHERE Country = 'Mexico';
      

Updating All Records (Caution!)

This example shows what happens when you omit the WHERE clause—all records are updated! This is dangerous and should be avoided unless you intend to change every single row in the table.

UPDATE Customers
SET ContactName = 'Juan';