Make rows immutable and allow insert

Question

I want to make the rows in a MySQL table immutable, once they are inserted I don't want it to be possible to change them. How can I do this?

(Ideally returning a suitable error message if any change is attempted).

I dont have grant privileges, so I would like to understand how to write BEFORE UPDATE trigger that raises an error (using signals or issuing a statement that always fails).

I use MySQL v5.

Answer 1

A very simple trigger should do it;

CREATE TRIGGER stop_update BEFORE UPDATE ON table1
FOR EACH ROW
  SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Update not allowed!'
//

An SQLfiddle to test with. You can add an update to test.

Of course, you may want to do the same for DELETE.

EDIT: If you're using a lower MySQL version than 5.5 that added signal, you can limit write by (not quite as cleanly) intentionally causing an error instead;

CREATE TRIGGER stop_update BEFORE UPDATE ON table1
FOR EACH ROW
  UPDATE UPDATE_OF_TABLE1_IS_NOT_ALLOWED SET value='Update not allowed!'
//

Another SQLfiddle. Can't add errors in the DDL and have SQLfiddle save it, so changing the updated id to 1 (an existing row) in the left window will fail the update.

Answer 2

Another solution without using triggers if you are using InnoDB.

Create another table which is its duplicate. The columns in new one should have foreign keys (hence the InnoDB requirement in this solution) that points to the columns of the original table in question.

Put a restriction like ON UPDATE RESTRICT for each key. Below code for one column:

CREATE TABLE original (
     ....
     immutable_column ...
     INDEX index1(immutable_column)
     ....
) ENGINE=INNODB;

CREATE TABLE restricter (
     .....
     col1,
     INDEX index2(col1),
     FOREIGN KEY (col1) REFERENCES original (immutable_colum) ON UPDATE RESTRICT ON DELETE CASCADE
) ENGINE=INNODB;

You do have to manually insert into this new table though.