In mysql we have — , \/* and \/*! comments. This post is mainly about very basic c-style comments.<\/p>\n\n\n\n
We normally see comments in MySQLdump as follows:<\/p>\n\n\n\n
\/*!40000 ALTER TABLE `a` DISABLE KEYS *\/;<\/pre>\n\n\n\n\/*!50013 DEFINER=`nitty_witty_com`@`%` SQL SECURITY DEFINER *\/<\/code><\/pre>\n\n\n\nThese are actually C-Style comments which has embeded sql and treated specially by MySQL server, but ignored by other database engines.
This helps in writing portable comments, it’s treated as a part of surrounding SQL by MySQL while ignored by other databases.<\/p>\n\n\n\nBut still you may wonder what are these numbers after \/*! – \/*!40000 or \/*!50013 !!<\/p>\n\n\n\n
There are two ways you can write the C-Style comments.
<\/strong> 1. \/*! SQL *\/<\/p>\n\n\n\nWhen SQL is embeded with comments [\/*! and *\/] it’s executed by MySQL server along with surrounding SQL.<\/p>\n\n\n\n
2. \/*!version-number SQL *\/<\/p>\n\n\n\n
When SQL is embeded with comment that begins with \/*! follwed by a version number, the SQL becomes version specific.
MySQL executes SQL body only if it server’s version is atleast as recent as version-number.<\/p>\n\n\n\nSo if your MySQL version is 5.x, following syntax will give error.<\/p>\n\n\n\n
SHOW \/*!60000 status *\/;<\/em><\/pre>\n\n\n\nbut following will work fine!<\/p>\n\n\n\n
SHOW \/*!50000 status *\/;<\/em><\/pre>\n\n\n\nSimilarly see below example:<\/span><\/span><\/span><\/p>\n\n\n\n
mysql> show \/*!50077 full*\/ tables;\n+----------------+------------+\n| Tables_in_test | Table_type |\n+----------------+------------+\n| comments | BASE TABLE |\n+----------------+------------+\n1 row in set (0.00 sec)<\/em><\/span><\/span><\/span><\/code><\/pre>\n\n\n\nmysql> show \/*!50078 full*\/ tables;
+----------------+
| Tables_in_test |
+----------------+
| comments |
+----------------+
1 row in set (0.00 sec)<\/em><\/pre>\n\n\n\nAbove code works at first attempt as MySQL Version was 5.0.77 but for version specific comment \/*!50078 it don’t.<\/em><\/p>\n\n\n\n
Further this can be used for adjusting the syntax or keywords change in latest releases. For eg MySQL 8.0.26 has replaced the word “slave” with “replica” in variables and you can use your query to hint the execution as:<\/p>\n\n\n\n
SELECT @@global.version as version, @@global.hostname as service \\\n\/*!80026 , @@global.replica_exec_mode as replica_exec_mode *\/ \\\n\/*!80026 , @@global.replica_skip_errors as replica_skip_errors<\/em> *\/\\G<\/code><\/pre>\n\n\n\nOther comments in MySQL:<\/h3>\n\n\n\n
\n
- — : double slash and a space or # : Hash<\/strong><\/li>\n<\/ul>\n\n\n\n
This is spanned till end of the line.<\/p>\n\n\n\n
\n
- \/* :<\/strong><\/li>\n<\/ul>\n\n\n\n
This is a normal multi-line comment.<\/p>\n","protected":false},"excerpt":{"rendered":"In mysql we have — , \/* and \/*! comments. This post is mainly about very basic c-style comments. \/*! : C-Style comments in MySQL We normally see comments in…\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[8,377],"tags":[547,549,238,681,27,427],"class_list":{"0":"post-667","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-mysql","7":"category-mysql-articles","8":"tag-547","9":"tag-80001-2","10":"tag-c-style-comments-in-mysqldump","11":"tag-comment-in-sql","12":"tag-comments","13":"tag-mysql"},"aioseo_notices":[],"amp_enabled":true,"_links":{"self":[{"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/posts\/667","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/comments?post=667"}],"version-history":[{"count":5,"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/posts\/667\/revisions"}],"predecessor-version":[{"id":3027,"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/posts\/667\/revisions\/3027"}],"wp:attachment":[{"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/media?parent=667"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/categories?post=667"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kedar.nitty-witty.com\/blog\/wp-json\/wp\/v2\/tags?post=667"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}