SQL · Theory

Table Locking

Theory

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Table Locking

LOCK TABLES students WRITE; stops others from writing that table until UNLOCK TABLES; This is old-school. InnoDB row locks inside transactions are the normal way: START TRANSACTION; UPDATE …; COMMIT;

READ lock lets others read. WRITE lock is exclusive. Easy to deadlock a lab if you forget UNLOCK.

Trap — LOCK TABLES as the answer for every concurrency question. Prefer transactions + InnoDB.

On the example next to this theory: Table Locking: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

Know LOCK TABLES exists. Prefer START TRANSACTION for InnoDB.

Table Locking — sample query

-- Table Locking
CREATE TABLE demo (
  id INTEGER PRIMARY KEY,
  label TEXT NOT NULL,
  qty INTEGER DEFAULT 0
);
INSERT INTO demo (id, label, qty) VALUES
  (1, 'alpha', 2),
  (2, 'beta', 5);
SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

Table Locking: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Short notes

DefLOCK TABLES … READ/WRITE then UNLOCK TABLES.
RulePrefer InnoDB transactions for real apps.
TrapForgetting UNLOCK TABLES.

-- Table Locking CREATE TABLE demo ( id INTEGER PRIMARY KEY, label TEXT NOT NULL, qty INTEGER DEFAULT 0 ); INSERT INTO demo (id, label, qty) VALUES (1, 'alpha', 2), (2, 'beta', 5); SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

Table Locking

Table Locking — sample query

Short notes

Questions

Table Locking

Table Locking — sample query

Short notes

Questions