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    <p><strong>Triggers</strong> - a trigger is a piece of SQL to execute either before or after an update, insert, or delete in a database. An example of a trigger in plain English might be something like: before updating a customer record, save a copy of the current record. Which would look something like:</p> <pre><code>CREATE TRIGGER triggerName AFTER UPDATE INSERT INTO CustomerLog (blah, blah, blah) SELECT blah, blah, blah FROM deleted </code></pre> <hr> <p>The difference between assertions and checks is a little more murky, many databases don't even support assertions.</p> <p><strong>Check Constraint</strong> - A check is a piece of SQL which makes sure a condition is satisfied before action can be taken on a record. In plain English this would be something like: All customers must have an account balance of at least $100 in their account. Which would look something like:</p> <pre><code>ALTER TABLE accounts ADD CONSTRAINT CK_minimumBalance CHECK (balance &gt;= 100) </code></pre> <p>Any attempt to insert a value in the balance column of less than 100 would throw an error.</p> <p><strong>Assertions</strong> - An assertion is a piece of SQL which makes sure a condition is satisfied or it stops action being taken on a <strong>database object</strong>. It could mean locking out the whole table or even the whole database.</p> <p>To make matters more confusing - a trigger could be used to enforce a check constraint and in some DBs can take the place of an assertion (by allowing you to run code un-related to the table being modified). A common mistake for beginners is to use a check constraint when a trigger is required or a trigger when a check constraint is required.</p> <p>An example: All new customers opening an account must have a balance of $100; however, once the account is opened their balance can fall below that amount. In this case you have to use a trigger because you only want the condition evaluated when a new record is inserted.</p>
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