Comment by billfruit
7 years ago
I know this is quite unrelated, and based on hazy memory of things from almost 10 years ago, DML statements in databases especially 'insert into' statements are not idempotent as I remember; ie if you try to select a few rows from a table table1 and insert them into table2 with same schema, if there were any identical rows already in table2, then whole insert will fail. My thinking at that time was that if these insert operations were idempotent, then there would be no need to explicitly check for duplicates before the insert.
At least in MySQL you can use “ON DUPLICATE KEY” to either ignore such things or optionally execute an update statement to change something about the matching row.
There is also REPLACE INTO
Assuming you have a relevant unique key setup of course.
> DML statements in databases especially 'insert into' statements are not idempotent as I remember;
GET is actually supposed to be safe which is stronger than idempotent; the SQL command that most naturally corresponds to GET—SELECT—is normally safe, but DML inherently is not.
But, sure, that INSERT isn't safe increases the amount of code needed to implement idempotent PUTs.