{"id":1126,"date":"2021-02-12T07:00:00","date_gmt":"2021-02-12T04:00:00","guid":{"rendered":"https:\/\/www.dataplatform.gr\/?p=1126"},"modified":"2024-12-03T21:05:17","modified_gmt":"2024-12-03T18:05:17","slug":"ti-einai-ta-triggers","status":"publish","type":"post","link":"https:\/\/www.dataplatform.gr\/en\/ti-einai-ta-triggers\/","title":{"rendered":"What are triggers?"},"content":{"rendered":"<p>Do they have overhead? Where is their utility? and finally how are they made?<\/p>\n\n\n\n<p>There are 3 different types: <strong>DDL triggers<\/strong> for CREATE \/ DROP actions, <strong>AFTER LOGON triggers <\/strong>for when a user logs in <strong>DML triggers<\/strong> for Update, Insert, Delete actions.<\/p>\n\n\n\n<p>In the article we will deal with DML triggers.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">What are DML triggers?<\/h2>\n\n\n\n<p>It is a special procedure that runs when a user tries to do a DML action (UPDATE, INSERT, DELETE) on the created table or view.<\/p>\n\n\n\n<p>Its purpose when activated varies, we may want it to raise an error, send an email, log records to another table and many more.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">How are they made?<\/h2>\n\n\n\n<p>We make a table with 3 entries:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">IF NOT EXISTS(select * from sysobjects where name='test')\ncreate table test ( onoma varchar(10), tilefono varchar(20))\nGO\n\ninsert into test values('Stratos','210101010\u2032)\n\ninsert into test values('Georgos','213920193\u2032)\n\ninsert into test values('Nikos','210101010\u2032)<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>Commands completed successfully.\n(1 row affected)\n(1 row affected)\n(1 row affected)\nTotal execution time: 00:00:00.059<\/code><\/pre>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">select onoma,tilefono from test;<\/pre>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"270\" height=\"211\" src=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/1-tr.png\" alt=\"\" class=\"wp-image-1128\"\/><\/figure>\n\n\n\n<p>We create a table that will be filled by the trigger:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">IF NOT EXISTS(select * from sysobjects where name='triggerINFO')\ncreate table triggerINFO(onoma varchar(100),tilefono varchar(100),pote_Svistike datetime)<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>Commands completed successfully.\nTotal execution time: 00:00:00.013<\/code><\/pre>\n\n\n\n<p>It&#039;s time to make our triggers.<\/p>\n\n\n\n<p>When creating a trigger we must always take into account the 3 cases depending on which action we want it to activate:<\/p>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>\/*\n\nFor INSERT operation, the INSERTED table will store the records being inserted\n\nfor DELETE operation, the DELETED table will store the records being deleted\n\nfor UPDATE operation, the DELETED table will store the original value of the records being updated and INSERTED stores the new value\n\n*\/<\/code><\/pre>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">create trigger triggerfordelete\non dbo.test \nfor delete\nas\ninsert into triggerinfo(onoma,tilefono,pote_Svistike)\nselect d.onoma,d.tilefono,SYSDATETIME()\nfrom deleted d\nwhere  tilefono like '210%'<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>Commands completed successfully.\n\nTotal execution time: 00:00:00.019<\/code><\/pre>\n\n\n\n<p>Now that we have made it, let&#039;s test it by deleting a record from the table:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">delete from test where onoma='Stratos'<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>(1 row affected)\n(1 row affected)\nTotal execution time: 00:00:00.012<\/code><\/pre>\n\n\n\n<p>We see we have the value that the user deleted stored in the other table:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">select * from triggerinfo<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>(1 row affected)\nTotal execution time: 00:00:00.016<\/code><\/pre>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"373\" height=\"75\" src=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/2-tr.png\" alt=\"\" class=\"wp-image-1129\" srcset=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/2-tr.png 373w, https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/2-tr-300x60.png 300w\" sizes=\"auto, (max-width: 373px) 100vw, 373px\" \/><\/figure>\n\n\n\n<p>Let&#039;s make another one in the update table:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">create trigger triggerforupdate\non dbo.test \nfor update\nas\ninsert into triggerinfo(onoma,tilefono,pote_Svistike)\nselect d.onoma,d.tilefono,SYSDATETIME()\nfrom deleted d<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>Commands completed successfully.\nTotal execution time: 00:00:00.018<\/code><\/pre>\n\n\n\n<p>Let&#039;s do our test:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">update test set tilefono = '2251020491' where onoma = 'Georgos'<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>(1 row affected)\n(1 row affected)\nTotal execution time: 00:00:00.005<\/code><\/pre>\n\n\n\n<p>As you will see in this example it produced 2 results. Why;;;<\/p>\n\n\n\n<p>Because very simply, the first result has the value recorded before it is changed in the INFO table by the trigger.<\/p>\n\n\n\n<p>In the second line you find the result of the update:<\/p>\n\n\n\n<pre class=\"EnlighterJSRAW\" data-enlighter-language=\"sql\" data-enlighter-theme=\"\" data-enlighter-highlight=\"\" data-enlighter-linenumbers=\"\" data-enlighter-lineoffset=\"\" data-enlighter-title=\"\" data-enlighter-group=\"\" data-no-translation=\"\" data-no-auto-translation=\"\">select * from triggerinfo<\/pre>\n\n\n\n<pre class=\"wp-block-code\" data-no-translation=\"\" data-no-auto-translation=\"\"><code>(2 rows affected)\nTotal execution time: 00:00:00.016<\/code><\/pre>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"393\" height=\"88\" src=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/3-tr.png\" alt=\"\" class=\"wp-image-1127\" srcset=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/3-tr.png 393w, https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/3-tr-300x67.png 300w\" sizes=\"auto, (max-width: 393px) 100vw, 393px\" \/><\/figure>\n\n\n\n<p>As we can see, this time it kept the value that the table had before it was updated due to the selection in the trigger for update.<\/p>\n\n\n\n<p>Triggers are useful tools but like most features it comes with overhead and it&#039;s not a little.<\/p>","protected":false},"excerpt":{"rendered":"<p>Do they have overhead? What is their usefulness? and finally how are they created? There are 3 different types: DDL triggers for CREATE \/ DROP actions, AFTER LOGON triggers for when a user logs in DML triggers for Update, Insert, Delete actions. In the article we will deal with DML triggers. What are DML triggers? It is a special procedure that runs [\u2026]<\/p>","protected":false},"author":1,"featured_media":700,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11,15],"tags":[29,39,60],"class_list":["post-1126","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-databases","category-ms-sqlserver","tag-databases","tag-sql","tag-triggers"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>\u03a4\u03b9 \u03b5\u03af\u03bd\u03b1\u03b9 \u03c4\u03b1 triggers - DataPlatform.gr<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.dataplatform.gr\/en\/ti-einai-ta-triggers\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"\u03a4\u03b9 \u03b5\u03af\u03bd\u03b1\u03b9 \u03c4\u03b1 triggers - DataPlatform.gr\" \/>\n<meta property=\"og:description\" content=\"\u0388\u03c7\u03bf\u03c5\u03bd overhead; \u03a0\u03bf\u03c5 \u03b5\u03af\u03bd\u03b1\u03b9 \u03b7 \u03c7\u03c1\u03b7\u03c3\u03b9\u03bc\u03cc\u03c4\u03b7\u03c4\u03b1 \u03c4\u03bf\u03c5\u03c2; \u03ba\u03b1\u03b9 \u03c4\u03ad\u03bb\u03bf\u03c2 \u03c0\u03c9\u03c2 \u03c6\u03c4\u03b9\u03ac\u03c7\u03bd\u03bf\u03bd\u03c4\u03b1\u03b9; \u03a5\u03c0\u03ac\u03c1\u03c7\u03bf\u03c5\u03bd 3 \u03b4\u03b9\u03b1\u03c6\u03bf\u03c1\u03b5\u03c4\u03b9\u03ba\u03ac \u03b5\u03af\u03b4\u03b7: DDL triggers \u03b3\u03b9\u03b1 actions CREATE \/ DROP, AFTER LOGON triggers \u03b3\u03b9\u03b1 \u03cc\u03c4\u03b1\u03bd \u03c3\u03c5\u03bd\u03b4\u03ad\u03b5\u03c4\u03b1\u03b9 \u03ad\u03bd\u03b1\u03c2 \u03c7\u03c1\u03ae\u03c3\u03c4\u03b7\u03c2 DML triggers \u03b3\u03b9\u03b1 actions Update, Insert, Delete. \u03a3\u03c4\u03bf \u03ac\u03c1\u03b8\u03c1\u03bf \u03b8\u03b1 \u03b1\u03c3\u03c7\u03bf\u03bb\u03b7\u03b8\u03bf\u03cd\u03bc\u03b5 \u03bc\u03b5 DML triggers. \u03a4\u03b9 \u03b5\u03af\u03bd\u03b1\u03b9 \u03c4\u03b1 DML triggers \u0395\u03af\u03bd\u03b1\u03b9 \u03bc\u03b9\u03b1 \u03b5\u03b9\u03b4\u03b9\u03ba\u03ae procedure \u03c0\u03bf\u03c5 \u03c4\u03c1\u03ad\u03c7\u03b5\u03b9 [&hellip;]\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.dataplatform.gr\/en\/ti-einai-ta-triggers\/\" \/>\n<meta property=\"og:site_name\" content=\"DataPlatform.gr\" \/>\n<meta property=\"article:publisher\" content=\"https:\/\/www.facebook.com\/dataplatform.gr\/\" \/>\n<meta property=\"article:published_time\" content=\"2021-02-12T04:00:00+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2024-12-03T18:05:17+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.dataplatform.gr\/wp-content\/uploads\/2020\/06\/dp_sql.png\" \/>\n\t<meta property=\"og:image:width\" content=\"1280\" \/>\n\t<meta property=\"og:image:height\" content=\"720\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"Stratos Matzouranis\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Stratos Matzouranis\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"4 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/\"},\"author\":{\"name\":\"Stratos Matzouranis\",\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/#\\\/schema\\\/person\\\/e87bf4fd02b65cb6aa0942f87245bbaf\"},\"headline\":\"\u03a4\u03b9 \u03b5\u03af\u03bd\u03b1\u03b9 \u03c4\u03b1 triggers\",\"datePublished\":\"2021-02-12T04:00:00+00:00\",\"dateModified\":\"2024-12-03T18:05:17+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/\"},\"wordCount\":56,\"commentCount\":0,\"publisher\":{\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/#organization\"},\"image\":{\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/www.dataplatform.gr\\\/wp-content\\\/uploads\\\/2020\\\/06\\\/dp_sql.png\",\"keywords\":[\"Databases\",\"SQL\",\"Triggers\"],\"articleSection\":[\"Databases\",\"Microsoft SQL Server\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/\",\"url\":\"https:\\\/\\\/www.dataplatform.gr\\\/ti-einai-ta-triggers\\\/\",\"name\":\"\u03a4\u03b9 \u03b5\u03af\u03bd\u03b1\u03b9 \u03c4\u03b1 triggers - 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