DML Notes

Trigger Basics: BEFORE, AFTER, INSERT, UPDATE and DELETE

Code the database runs automatically when data changes. Learn the six trigger points, the NEW and OLD row references, and how to write an audit trigge...

SQL

Finding and Removing Duplicates

Detect duplicate rows on any definition of "duplicate", inspect them safely, delete all but one, and add the constraint that stops them coming back.

SQL

DELETE and Safe Data Modification

DELETE removes rows permanently. Learn targeted deletes, deleting through a join, soft deletes and the habits that stop a one line mistake becoming an...

SQL

UPDATE: Conditional and Multi Column Changes

UPDATE changes existing rows. Learn multi column updates, conditional logic with CASE, updates driven by a join and how to check the blast radius firs...

SQL

INSERT: Single Row, Multi Row and INSERT SELECT

Every way to put rows into a table: explicit column lists, multi row VALUES, INSERT ... SELECT, defaults and handling duplicate keys.

SQL

DROP, TRUNCATE and DELETE Compared

Three ways to remove data, with very different consequences. Learn what each one destroys, what can be rolled back and what resets identity counters.