UPDATE Query

The UPDATE statement in SQL is used to modify existing data in a table.

You can use UPDATE to:

• Correct mistakes in data
• Change values when new information is available
• Update multiple rows at once based on a condition

Basic Syntax

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

• SET: specifies which columns you want to update and the new values

• WHERE: filters which rows will be updated

  • If you omit WHERE, all rows in the table will be updated

Example Table: students

Example 1: Update One Row

Change the age of Bob Johnson to 23:

UPDATE students
SET age = 23
WHERE name = 'Bob Johnson';

Example 2: Update Multiple Columns

Change both the age and email for Alice Smith:

UPDATE students
SET age = 21,
    email = 'alice.smith@example.com'
WHERE id = 1;

Example 3: Update Multiple Rows

Increase the age of all students who are younger than 21:

UPDATE students
SET age = age + 1
WHERE age < 21;

Example 4: Update All Rows (No WHERE)

Caution: If you leave out WHERE, all rows in the table will be updated:

UPDATE students
SET enrollment_date = '2025-06-30';

This will set the same enrollment date for every student in the table.

Important Notes

• Always use a WHERE clause unless you intentionally want to update every row.
• You can update one column or multiple columns in the same query.
• You can use expressions in SET, such as age = age + 1.

Summary

• UPDATE modifies existing data in a table.
• SET defines the new values for one or more columns.
• WHERE filters which rows are updated — do not forget this.
• You can update one row or many rows at once.