MySQL Triggers for Data Integrity – In lieu of Check Constraints Part 2.

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Photo by Todd Quackenbush on Unsplash

Part 1 of this two-part series, discussed the requirements for implementing data integrity checks using a TRIGGER. With the success of that TRIGGER, this second blog post will dive into its workings, gaining an understanding of how the checks are performed, along with the TRIGGER itself.


Note: All data, names or naming found within the database presented in this post, are strictly used for practice, learning, instruction, and testing purposes. It by no means depicts actual data belonging to or being used by any party or organization.

I will be using Xubuntu Linux 16.04.3 LTS (Xenial Xerus) and MySQL 5.7.21 for these exercises.

Nuts and Bolts

What is a TRIGGER?

A TRIGGER is a database object that is basically a special kind of procedure, called implicitly due to some database event.

To start, let’s revisit the collect_to_stage TRIGGER.

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DELIMITER $$
CREATE TRIGGER collect_to_stage BEFORE INSERT
ON asset_staging
FOR EACH ROW
BEGIN
  IF NEW.kind = 0 THEN
  SET NEW.kind = 'other';
  INSERT INTO flagged_asset(f_pipe_name, f_pipe_jt_num, f_pipe_heat,
    f_pipe_length, f_defect, f_message, import_date)
  VALUES(NEW.pipe_name, NEW.pipe_jt_num, NEW.pipe_heat, NEW.pipe_length,
    'Unknown Type', CONCAT(NEW.pipe_name, ' was not in the acceptable list.'), NOW());
  END IF;
  IF NEW.pipe_length < = 0 THEN
  INSERT INTO flagged_asset(f_pipe_name, f_pipe_jt_num, f_pipe_heat,
    f_pipe_length, f_defect, f_message, import_date)
  VALUES(NEW.pipe_name, NEW.pipe_jt_num, NEW.pipe_heat, NEW.pipe_length,
    'Questionable Length', CONCAT(NEW.pipe_name, ' needs the length verified for accuracy.'), NOW());
  END IF;
END $$
DELIMITER ;

How does this TRIGGER perform its required work?
Let’s find out line by line.

    • Line 1: DELIMITER $$ – Line’s 1 and 20 share the same keyword, yet its implications are different. The commonality lies in setting the DELIMITER. The difference is in the characters used. Why redefine it to begin with?
      By default, the statement DELIMITER recognized by the mysql command-line program is the semicolon (;). In order to use it (the semicolon) in multiple statements, requires an alternative character.
    • Line 2: CREATE TRIGGER collect_to_stage BEFORE INSERT – Line 2 specifies the TRIGGER timing , indicating it will fire BEFORE an INSERT operation.
    • Line 3: ON asset_staging – The target table.
    • Line 4: FOR EACH ROW – Indicates the TRIGGER actuates, occurring for each row affected by the event condition. In this case, INSERT.
    • Line 5: BEGIN – Used to start a section of compound statements.
    • Line 6: IF NEW.kind = 0 THEN – Line 6 is vital to the kind column integrity check within an IF conditional block. The ENUM data type is a string object composed of a permitted list of values assigned at creation. Each value has an index. The index for an invalid value is 0 (zero). With this information, we can check the NEW.kind column value. If the value is equal to 0, then an invalid value has been stored for this column.
      Depending on the type of TRIGGER event, NEW and OLD keywords, enable access to row column values. It is worth noting, INSERT operations provide only a NEW.column value.
    • Line 7: SET NEW.kind = 'other'; – Here, I am setting the unacceptable kind column value to ‘other’ until it can be investigated, determined what the correct type of asset it is, and corrected. The SET statement is purposely located within this IF block. By locating it here, setting the consistent ‘other’ value only happens if an invalid value has been imported. Therefore, we still retain the acceptable values for those records having them.
    • Lines 8 – 11:
       INSERT INTO flagged_asset(f_pipe_name, f_pipe_jt_num, f_pipe_heat, f_pipe_length, f_defect, f_message)
              VALUES(NEW.pipe_name, NEW.pipe_jt_num, NEW.pipe_heat, NEW.pipe_length,
          'Unknown Type', CONCAT(NEW.pipe_name, ' was not in the acceptable list.'));

      This INSERT statement stores the record in table flagged_asset due to failing the column check.

    • Line 12: END IF; – The end of this conditional block.
    • Line 13: IF NEW.pipe_length < = 0 THEN – Beginning this second check inside an IF conditional block. If the pipe_length value is either 0 (zero) or less, it is flagged. Simple enough.
  • Line 14 – 17:
     INSERT INTO flagged_asset(f_pipe_name, f_pipe_jt_num, f_pipe_heat, f_pipe_length, f_defect, f_message)
      VALUES(NEW.pipe_name, NEW.pipe_jt_num, NEW.pipe_heat, NEW.pipe_length,
        'Questionable Length', CONCAT(NEW.pipe_name, ' needs the length verified for accuracy.'));

    Records not passing the ‘length check’ are inserted into table flagged_asset.

  • Line 18: END IF; – Ends this IF conditional block.
  • Line 19: END $$ – Ending this BEGIN...END construct and executing the trigger with $$ which is the delimiter. For now.
  • Line 20: DELIMITER ; – Re-setting the delimiter back to the default semicolon (;)

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    Final Thoughts

    From enforcing business rules to ensuring the integrity of your data, triggers have their place in databases. Oftentimes, they are used to maintain logs of events.
    Although not discussed in this series, MySQL does allow a table to have multiple triggers with the same timing and event action, determining firing order using PRECEDES or FOLLOWS followed by the existing TRIGGER.


    That’s A Wrap

    Wow! This 2 part series on triggers has blown my mind! I have learned so much with this concept of their use and I hope you have as well. I’d love to hear any feedback on triggers from those more experienced with them.

    Explore the official MySQL 5.7 Online Manual for an in-depth overview of all topics covered in this post.


    A Call To Action!

    Thank you for taking the time to read this post. I truly hope you discovered something interesting and enlightening. Please share your findings here, with someone else you know who would get the same value out of it as well.

    Have I mentioned how much I love a cup of coffee?!?!

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    Be sure and visit the “Best Of” page for a collection of my best blog posts.


    Josh Otwell

    Josh Otwell has a passion to study and grow as a SQL Developer and blogger. Other favorite activities find him with his nose buried in a good book, article, or the Linux command line. Among those, he shares a love of tabletop RPG games, reading fantasy novels, and spending time with his wife and two daughters.


    Disclaimer: The examples presented in this post are hypothetical ideas of how to achieve similar types of results. They are not the utmost best solution(s). Your particular goals and needs may vary. Use those practices that best benefit your needs and goals. Opinions are my own.