SQL · Theory

DELETE Record

Theory

34/439

DELETE Record

DELETE FROM students WHERE roll = 12; removes matching rows. The table remains. DELETE FROM students; with no WHERE deletes everyone — same danger as UPDATE.

FOREIGN KEY: you cannot delete Asha if marks rows still point to her roll, unless ON DELETE CASCADE. That is a feature. Don’t disable FK to ‘make DELETE work’ in production.

Trap — DELETE vs DROP vs TRUNCATE mixed up. DELETE = rows. TRUNCATE = all rows, keep table. DROP = table gone.

On the example next to this theory: DELETE Record — dELETE removes matching rows. TRUNCATE clears a whole table faster (MySQL).

Exam tip

DELETE FROM students WHERE roll = 12;

DELETE Record — sample query

CREATE TABLE tags (id INTEGER PRIMARY KEY, label TEXT);
INSERT INTO tags VALUES (1, 'old'), (2, 'keep');
DELETE FROM tags WHERE label = 'old';
SELECT * FROM tags;

DELETE Record — dELETE removes matching rows. TRUNCATE clears a whole table faster (MySQL).

Short notes

DefDELETE FROM t WHERE …; removes rows.
RuleWHERE on the key. FKs may block the delete.
TrapDELETE / TRUNCATE / DROP confusion.

SQL · Theory

DELETE Record

← All stacks

Theory

34/439

DELETE Record

DELETE FROM students WHERE roll = 12; removes matching rows. The table remains. DELETE FROM students; with no WHERE deletes everyone — same danger as UPDATE.

FOREIGN KEY: you cannot delete Asha if marks rows still point to her roll, unless ON DELETE CASCADE. That is a feature. Don’t disable FK to ‘make DELETE work’ in production.

Trap — DELETE vs DROP vs TRUNCATE mixed up. DELETE = rows. TRUNCATE = all rows, keep table. DROP = table gone.

On the example next to this theory: DELETE Record — dELETE removes matching rows. TRUNCATE clears a whole table faster (MySQL).

Exam tip

DELETE FROM students WHERE roll = 12;

DELETE Record — sample query

CREATE TABLE tags (id INTEGER PRIMARY KEY, label TEXT);
INSERT INTO tags VALUES (1, 'old'), (2, 'keep');
DELETE FROM tags WHERE label = 'old';
SELECT * FROM tags;

DELETE Record — dELETE removes matching rows. TRUNCATE clears a whole table faster (MySQL).

Short notes

DefDELETE FROM t WHERE …; removes rows.
RuleWHERE on the key. FKs may block the delete.
TrapDELETE / TRUNCATE / DROP confusion.

DELETE Record

DELETE Record — sample query

Short notes

Questions

DELETE Record

DELETE Record — sample query

Short notes

Questions