Compare locking strategies.
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Understand the problem
locking
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03
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Read the code
Optimistic update (version check)
-- Optimistic locking with a version column (compare-and-set).
-- 1. Read the current row + its version:
SELECT id, stock, version FROM products WHERE id = 42; -- version = 7
-- 2. Write only if nobody changed it in the meantime:
UPDATE products
SET stock = stock - 1, version = version + 1
WHERE id = 42 AND version = 7;
-- If 0 rows were updated, someone else won the race -> re-read and retry.05
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