Comments - Using Compound Statements Outside of Stored Programs
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This doesn't explain whether a compound statement outside an explicit transaction is treated as an atomic statement or not.
e.g. It's not clear if this can fail to return a row:
IF (EXISTS (SELECT * FROM table WHERE id=1)) THEN SELECT data FROM table WHERE id=1;ELSE SELECT 'data does not exist';END IF;
...if the EXISTS() is executed in a separate transaction to the subsequent SELECT, it might.
The syntax
BEGIN NOT ATOMICkind of implies that it's not treated as an atomic statement. Other compound statements, likeIFdon't say "not atomic" explicitly, but they are also not atomic. For an atomic compound statement one has to useBEGIN ATOMIC, which is not supported as of now.