Isolation levels.

also: isolation level · read committed · repeatable read · serializable isolation · snapshot isolation · read uncommitted

How much concurrent transactions can see of each other: read uncommitted, read committed, repeatable read and serializable, weakest to strongest.

the two-minute refresher

Read uncommitted allows dirty reads of data that may roll back.
Read committed stops dirty reads but allows non-repeatable reads.
Repeatable read keeps a row stable within a transaction; serializable behaves as if transactions ran one at a time.
Stronger isolation costs throughput: pick the weakest level your invariants survive.

also: isolation level · read committed · repeatable read · serializable isolation · snapshot isolation · read uncommitted

How much concurrent transactions can see of each other: read uncommitted, read committed, repeatable read and serializable, weakest to strongest.

Read uncommitted allows dirty reads of data that may roll back.
Read committed stops dirty reads but allows non-repeatable reads.
Repeatable read keeps a row stable within a transaction; serializable behaves as if transactions ran one at a time.
Stronger isolation costs throughput: pick the weakest level your invariants survive.