Update Multiple Rows in MySQL

Updating many rows

A single UPDATE can change many rows at once — you do not need one statement per row. With a WHERE clause it targets a group, with expressions it can compute new values, and with CASE it can give each row a different value. Just always update safely.

Update by condition

UPDATE students
SET marks = marks + 5
WHERE class = 10;

Every student in class 10 gets 5 bonus marks in one statement. Notice marks = marks + 5 uses each row's current value to compute the new one.

Update several columns

UPDATE employees
SET salary = salary * 1.1, status = 'reviewed'
WHERE department = 'Sales';

You can set multiple columns in one UPDATE, separated by commas. Here Sales employees get a 10% raise and a status change together.

Different values with CASE

UPDATE students
SET grade = CASE
    WHEN marks >= 90 THEN 'A'
    WHEN marks >= 75 THEN 'B'
    ELSE 'C'
END;

A CASE in the SET clause gives each row a value based on its own data. Here every student's grade is set from their marks — all in one statement.

Update from a join

UPDATE orders o
JOIN customers c ON o.customer_id = c.id
SET o.city = c.city;

An UPDATE with a JOIN copies values from a related table. Here each order's city is filled from its customer's city, matched by the key — no manual lookups.

Doing it safely

• Always use WHERE (unless you truly mean all rows).
• Test first with a SELECT using the same condition.
• Use a transaction for risky updates, so you can ROLLBACK.

These habits prevent the classic disaster of updating every row by mistake.

Common mistakes

• Omitting WHERE. Then every row is updated.
• Forgetting END in CASE. A CASE must finish with END.
• Not testing with SELECT. Preview the rows before updating.
• Skipping transactions on big updates. They let you undo mistakes.

Summary

• One UPDATE with WHERE changes all matching rows at once.
• Use expressions like marks + 5 to compute from current values.
• Set several columns in one statement, separated by commas.
• Use CASE to give each row a different value.
• Update from a JOIN, and always update safely with WHERE and testing.

Next, run these safely with safe UPDATE and DELETE, or add many rows in inserting multiple rows.