SQL · Theory

UPDATE Record

Theory

33/439

UPDATE Record

UPDATE students SET marks = 80 WHERE roll = 12; changes matching rows. SET is what to change. WHERE is which rows. Without WHERE, every student becomes 80. Workbench safe-updates mode blocks UPDATE/DELETE without a key — that is a seatbelt, not a bug.

Preview: SELECT * FROM students WHERE roll = 12; then UPDATE. In a transaction: START TRANSACTION; UPDATE …; SELECT to check; COMMIT or ROLLBACK.

Trap — WHERE name = 'Asha' when two Ashas exist. Use the PRIMARY KEY. Trap: marks = marks + 1 without WHERE.

On the example next to this theory — UPDATE Record — uPDATE changes existing rows — always scope with WHERE.

Exam tip

UPDATE … SET … WHERE roll = 12. Never skip WHERE.

UPDATE Record — sample query

CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, price REAL);
INSERT INTO products VALUES (1, 'Mouse', 499);
UPDATE products SET price = 449 WHERE id = 1;
SELECT * FROM products;

UPDATE Record — uPDATE changes existing rows — always scope with WHERE.

Short notes

DefUPDATE t SET col = val WHERE key = …;
RuleAlways WHERE on a key. Preview with SELECT.
TrapMissing WHERE updates the whole table.

SQL · Theory

UPDATE Record

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Theory

33/439

UPDATE Record

Preview: SELECT * FROM students WHERE roll = 12; then UPDATE. In a transaction: START TRANSACTION; UPDATE …; SELECT to check; COMMIT or ROLLBACK.

Trap — WHERE name = 'Asha' when two Ashas exist. Use the PRIMARY KEY. Trap: marks = marks + 1 without WHERE.

On the example next to this theory — UPDATE Record — uPDATE changes existing rows — always scope with WHERE.

Exam tip

UPDATE … SET … WHERE roll = 12. Never skip WHERE.

UPDATE Record — sample query

CREATE TABLE products (id INTEGER PRIMARY KEY, name TEXT, price REAL);
INSERT INTO products VALUES (1, 'Mouse', 499);
UPDATE products SET price = 449 WHERE id = 1;
SELECT * FROM products;

UPDATE Record — uPDATE changes existing rows — always scope with WHERE.

Short notes

DefUPDATE t SET col = val WHERE key = …;
RuleAlways WHERE on a key. Preview with SELECT.
TrapMissing WHERE updates the whole table.

UPDATE Record

UPDATE Record — sample query

Short notes

Questions

UPDATE Record

UPDATE Record — sample query

Short notes

Questions