Question

Is there a way to restrict insert or update to specific columns in a table using innodb engine in MySQL? I want to update these columns only through triggers or procedures

What I want to do is to calculate these columns automatically and not allow the change of these directly. Let me know if my approach is not correct

Answer 1

Let me know if my approach is not correct

Your approach is unusual, but not impossible. Whether it is “correct” will depend on factors not mentioned in your question:

• If the columns are always the result of some simple expression, you could simply specify that expression in each SELECT:

  SELECT col_a, col_b, LEAST(10*col_a, 5*col_b) AS calculated_column
  FROM   mytbl
  WHERE  ...
  

  Or store the results in a view so that you don’t need to write the expression(s) every time:

  CREATE VIEW myview AS
  SELECT col_a, col_b, LEAST(10*col_a, 5*col_b) AS calculated_column
  FROM   mytbl
  

  In which case, to select the results one merely need do:

  SELECT * FROM myview WHERE ...
  

  However, note that calculated_column will still be evaluated each time you select from the view, so could lead to performance problems if it’s a complicated expression or your table is large. If instead you want to store the result of the calculation (so that it need not be evaluated on each SELECT), you could define triggers that overwrite any value supplied by the user:

  CREATE TRIGGER mytbl_insert AFTER INSERT ON mytbl FOR EACH ROW SET
    NEW.calculated_column = LEAST(10*NEW.col_a, 5*NEW.col_b);
  
  CREATE TRIGGER mytbl_update AFTER UPDATE ON mytbl FOR EACH ROW SET
    NEW.calculated_column = LEAST(10*NEW.col_a, 5*NEW.col_b);
  

• If the columns will actually be periodically updated, but only from a stored procedure:

  1. Deny your existing user(s) permission to update the columns:

     REVOKE INSERT (col1, col2), UPDATE (col1, col2) ON mydb.mytbl FROM myuser;
     

     Note that if your user has table- or database-level privileges, they will need to be revoked and replaced with column-level ones as appropriate.

  2. Create a user under which the procedure will run:

     GRANT INSERT, UPDATE ON mydb.mytbl TO
     'procuser'@'nonexistent' IDENTIFIED BY PASSWORD '';
     

  3. Define your procedure to run under that new user:

     CREATE DEFINER = 'procuser'@'nonexistent' PROCEDURE ...