UPDATE
UPDATE changes existing data in a table. Unlike INSERT, which adds new rows, UPDATE modifies rows that are already there. UPDATE students SET age = 17 WHERE id = 1; changes the age column to 17, but only for the row where id equals 1. The SET clause defines what changes; the WHERE clause defines which rows are affected. You can update multiple columns in one statement: UPDATE students SET age = 17, name = "Ali Khan" WHERE id = 1;, separating each column = value pair with a comma. This is the single most important habit in this entire topic: never run UPDATE without a WHERE clause, unless you genuinely intend to change every single row in the table. UPDATE students SET age = 17; with no WHERE sets every student's age to 17 a mistake that's easy to make and hard to undo.
Why this matters
- ·UPDATE lets you correct or refresh existing data without deleting and re-inserting it.
- ·SET can change several columns in a single, efficient statement.
- ·A precise WHERE clause lets you target exactly the row (or rows) that actually need changing.
Common mistakes
- ·Running UPDATE without a WHERE clause changes every row in the table one of the most damaging and common SQL mistakes.
- ·A WHERE condition that's broader than intended can update more rows than you meant to.
- ·There is no automatic undo always double-check an UPDATE's WHERE clause, ideally by running the same condition as a SELECT first.
Query OK, 1 row affected +----+------+-----+ | id | name | age | +----+------+-----+ | 1 | Ali | 17 | | 2 | Sara | 17 | +----+------+-----+ 2 rows in set
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Frequently asked questions
INSERT adds a brand-new row. UPDATE modifies values in a row (or rows) that already exist.
Practice questions
Question 1
Write an UPDATE that changes Sara's age to 18, using her id.
PHP runs on a server, not in the browser this shows the expected output instead of a live one.
Question 2
Write an UPDATE that changes both the name and age for the student with id 1.
PHP runs on a server, not in the browser this shows the expected output instead of a live one.
Quick check
Just for this topic answer honestly, then check yourself.
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