{"id":3644,"date":"2020-12-18T04:05:50","date_gmt":"2020-12-18T04:05:50","guid":{"rendered":"https:\/\/studysection.com\/blog\/?p=3644"},"modified":"2020-12-18T05:39:01","modified_gmt":"2020-12-18T05:39:01","slug":"introduction-to-triggers-and-views-in-sql","status":"publish","type":"post","link":"https:\/\/studysection.com\/blog\/introduction-to-triggers-and-views-in-sql\/","title":{"rendered":"Introduction to Triggers and Views in SQL"},"content":{"rendered":"<h2>Triggers:<\/h2>\n<p>Triggers are event-condition-action (ECA) rules. When an event occurs, test the condition if the condition is satisfied, execute the action.<\/p>\n<p><strong>Example:<\/strong><\/p>\n<ul>\n<li>Event: Save Percentage<\/li>\n<li>Condition: Marks in subjects is not equal to zero<\/li>\n<li>Action: Calculate the percentage of the student.<\/li>\n<\/ul>\n<p><strong>Trigger syntax and example:<\/strong><br \/>\n<code>create trigger student_marks<br \/>\nbefore INSERT<br \/>\non<br \/>\nStudents<br \/>\nfor each row<br \/>\nWHEN (Student.subj1>=0) AND (Student.subj2>=0)<br \/>\nset Student.total = Student.subj1 + Student.subj2, Student.percentage = Student.total * 60 \/ 100;<\/code><\/p>\n<p><strong>Trigger options<\/strong><br \/>\nPossible events include:<\/p>\n<ul>\n<li>INSERT ON table<\/li>\n<li>DELETE ON table<\/li>\n<li>UPDATE [OF column] ON table<\/li>\n<\/ul>\n<p><strong>Granularity\u2014trigger can be activated:<\/strong><\/p>\n<ul>\n<li>FOR EACH ROW modified<\/li>\n<li>FOR EACH STATEMENT that performs modification<\/li>\n<\/ul>\n<p><strong>Timing\u2014action can be executed:<\/strong><\/p>\n<ul>\n<li>AFTER or BEFORE the triggering event<\/li>\n<li>INSTEAD OF the triggering event on views7<\/li>\n<\/ul>\n<h3>VIEWS:<\/h3>\n<p>A view is sort of a \u201cvirtual\u201d table defined by a question, which describes the way to compute the view contents on the fly. DBMS stores the view definition query rather than view contents. <\/p>\n<p><strong>Creating and dropping views:<\/strong><br \/>\n<strong>To create a view<\/strong> for members of Jennifer\u2019s Circle<\/p>\n<p><code>CREATE VIEW JenCircle AS SELECT * FROM User WHERE userid IN (SELECT userid FROM Member WHERE groupid = 'jen');<\/code><\/p>\n<ul>\n<li>Tables (User and Member) used in defining a view are called \u201cbase tables\u201d<\/li>\n<\/ul>\n<p><strong>To drop a view<\/strong><\/p>\n<ul>\n<li>DROP VIEW JessicaCircle<\/li>\n<\/ul>\n<p><strong>Why use views?<\/strong><\/p>\n<ul>\n<li>To hide <a href=\"https:\/\/studysection.com\/blog\/why-everyone-is-worried-about-the-computer-or-data-security\/\">data<\/a> from users<\/li>\n<li>To hide complexity from users<\/li>\n<li>Logical data independence<\/li>\n<li>If applications affect views, we will change the underlying schema without affecting applications<\/li>\n<li>Recall physical data independence: we can change the physical data organization without affecting the applications related.<\/li>\n<li>A uniform interface can be provided to different implementations or sources.<\/li>\n<\/ul>\n<p>Real database applications use tons of views<\/p>\n<p><small><em>Get certification for your knowledge in the fundamentals of Computer functioning by clearing the Computer Certification exam conducted by StudySection. After going through this <a href=\"https:\/\/www.studysection.com\/computer-applications-diploma\">Computer Certification Exam<\/a>, you will be able to evaluate your basic knowledge of computers. <\/em><\/small><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Triggers: Triggers are event-condition-action (ECA) rules. When an event occurs, test the condition if the condition is satisfied, execute the<\/p>\n","protected":false},"author":1,"featured_media":3645,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[306,43],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v21.7 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>StudySection Blog - Introduction to Triggers and Views in SQL<\/title>\n<meta name=\"description\" content=\"Triggers are event-condition-action (ECA) rules. 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