{"id":584,"date":"2022-07-25T13:41:21","date_gmt":"2022-07-25T13:41:21","guid":{"rendered":"http:\/\/www.softwaretraininginchennai.com\/blog\/?p=584"},"modified":"2022-07-25T13:43:15","modified_gmt":"2022-07-25T13:43:15","slug":"sql-constraints","status":"publish","type":"post","link":"https:\/\/www.softwaretraininginchennai.com\/blog\/sql-constraints\/","title":{"rendered":"SQL Constraints"},"content":{"rendered":"<p><strong><u>SQL Constraints<\/u><\/strong><\/p>\n<p>Constraints are the rules that can be applied on the type of data in a table.\u00a0i.e, it can be used to limit the type of data that can go into a table.<\/p>\n<p>Constraints can be used either when the table is created, or after the table is created.<\/p>\n<p>It can be either applied to a column in a table or to the whole table.<\/p>\n<p>The constraints used in SQL are:<\/p>\n<ul>\n<li>NOT NULL<\/li>\n<li>UNIQUE<\/li>\n<li>PRIMARY KEY<\/li>\n<li>FOREIGN KEY<\/li>\n<li>CHECK<\/li>\n<li>DEFAULT<\/li>\n<li>CREATE INDEX<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p><strong>NOT NULL:<\/strong> if this constraint is used in any column, then it will not allow NULL values in that particular column anymore.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250)\u00a0NOT\u00a0NULL<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>In this command, all the columns are specified with NULL Constraint, so all the columns will allow NULL data in it.<\/em><\/p>\n<p>&nbsp;<\/p>\n<p><strong>UNIQUE:<\/strong> if this constraint is used in any column, then it does not allow to repeat any data or value inn that particular column anymore.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int\u00a0NOT\u00a0NULL UNIQUE,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250)\u00a0NOT\u00a0NULL<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>In this command, all the ID column is specified with UNIQUE Constraint, so that the ID cannot be repeated with same data.<\/em><\/p>\n<p>&nbsp;<\/p>\n<p><strong>PRIMARY KEY:<\/strong> This constraint uniquely identifies each record in a table or column and also does\u00a0 not allow NULL data in\u00a0 it.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250),<\/em><\/p>\n<p><em>PRIMARY KEY (ID)<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>In this command, the PRIMARY KEY Constraint is specified for ID, so that the ID cannot be repeated with same data, also ID cannot be NULL data.<\/em><\/p>\n<p>&nbsp;<\/p>\n<p><strong>FOREIGN KEY:<\/strong> The FOREIGN KEY Constraint references a row in another table, here, the table with the foreign key Constraint is called as the child table, and the table with the primary key is called the parent table.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>Parent Table::<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250),<\/em><\/p>\n<p><em>PRIMARY KEY (ID)<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>Child Table::<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0Marks (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Mark int\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Subcode int\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0PRIMARY\u00a0KEY\u00a0(Mark),<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0FOREIGN\u00a0KEY\u00a0(ID)\u00a0REFERENCES\u00a0StudentDetail(ID)<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>Here the \u201cMark&#8221; of a student in the &#8220;Table Marks&#8221; refers to the &#8220;ID&#8221;\u00a0 of the student in the &#8220;Table StudentDetail&#8221;.<\/em><\/p>\n<p>&nbsp;<\/p>\n<p><strong>CHECK:<\/strong> The CHECK Constraint validates condition for new value.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250)\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>CHECK\u00a0(Branch=&#8217;CSE&#8217;)<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>In this command, the CHECK Constraints, allows only the CSE as Branch.<\/em><\/p>\n<p>&nbsp;<\/p>\n<p><strong>DEFAULT:<\/strong> This Constraint is used to set a default value for any particular column, if any value is not specified.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250)\u00a0DEFAULT\u00a0&#8216;CSE&#8217;,<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>In this command, the DEFAULT Constraints, fills the Branch as CSE in default when Branch is not specified.<\/em><\/p>\n<p>&nbsp;<\/p>\n<p><strong>CREATE INDEX:<\/strong> This Constraint is used to speed-up the process, i.e it can create and retrieve any data from the database more quickly.<\/p>\n<p><em>Eg:<\/em><\/p>\n<p><em>CREATE\u00a0TABLE\u00a0StudentDetail (<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 ID int\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0\u00a0Name varchar(250)\u00a0NOT\u00a0NULL,<\/em><\/p>\n<p><em>\u00a0\u00a0\u00a0 Branch varchar(250)\u00a0NOT\u00a0NULL<\/em><\/p>\n<p><em>);<\/em><\/p>\n<p><em>CREATE\u00a0INDEX\u00a0ix_Branch<\/em><\/p>\n<p><em>ON\u00a0StudentDetail (Branch);<\/em><\/p>\n<p><em>Here, in the table StudentDetail, the CREATE INDEX command creates an index ix_Branch using the Branch column.<\/em><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>SQL Constraints Constraints are the rules that can be applied on the type of data in a table.\u00a0i.e, it can be used to limit the type of data that can go into a table. Constraints can be used either when the table is created, or after the table is created. It can be either applied [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2,235,234],"tags":[14,237,236],"class_list":["post-584","post","type-post","status-publish","format-standard","hentry","category-dot-net-training","category-rdbms","category-sql-server","tag-net-training-in-chennai","tag-sql-constraints","tag-sql-server-online-training"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.2 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>SQL Constraints | Maria Academy<\/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.softwaretraininginchennai.com\/blog\/sql-constraints\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"SQL Constraints | Maria Academy\" \/>\n<meta property=\"og:description\" content=\"SQL Constraints Constraints are the rules that can be applied on the type of data in a table.\u00a0i.e, it can be used to limit the type of data that can go into a table. 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