{"id":770,"date":"2019-11-21T15:19:22","date_gmt":"2019-11-21T09:49:22","guid":{"rendered":"https:\/\/binaryterms.com\/?p=770"},"modified":"2022-02-14T15:14:18","modified_gmt":"2022-02-14T09:44:18","slug":"null-value-in-sql","status":"publish","type":"post","link":"https:\/\/binaryterms.com\/null-value-in-sql.html","title":{"rendered":"Null Value in SQL"},"content":{"rendered":"<p><strong>Null Value<\/strong> in SQL is the special value, allocated to an <strong>attribute<\/strong> in a relation that indicates, the information regarding the corresponding attribute is either <strong>unknown<\/strong> or <strong>doesn\u2019t even exist<\/strong>. In this section, we will discuss the null values in detail.<\/p>\n<h2>What is the Null Value in SQL?<\/h2>\n<p>The <strong>null value<\/strong> assigned to an attribute in a relation specifies that the <strong>value<\/strong> for the corresponding <strong>attribute<\/strong> is either:<\/p>\n<ul>\n<li><strong>not known<\/strong> (i.e. value exist but we don\u2019t have any information regarding that value)<\/li>\n<li>the <strong>value<\/strong> for a certain attribute <strong>doesn\u2019t exist<\/strong> at all.<\/li>\n<\/ul>\n<p>When a new attribute is added to an already existing relation using <strong>alter table<\/strong> command then all the tuples of that relation initially have a <strong>null value<\/strong> for that newly added attribute.<\/p>\n<p>Well in certain cases, we restrict null value from being entered as a value to an attribute in a relation. Such as the <strong>primary key attribute value<\/strong> of a relation is required to be <strong>not null<\/strong>. Hence, the <strong>not-null constraint<\/strong> is added to the primary key attribute of the relation while it is created.<\/p>\n<p>Allocating null values to attributes make a relation difficult to handle. It is preferred not to make frequent use of them as they create difficulties when we access or update the database. In the further section, we will discuss the problems with null values.<\/p>\n<ol>\n<li><a href=\"#OperationsonNULLValues\">Operations on NULL Values<\/a><\/li>\n<li><a href=\"#ISNULL&amp;ISNOTNULL\">IS NULL &amp; IS NOT NULL<\/a><\/li>\n<li><a href=\"#INSERTNULLValues\">INSERT NULL Values<\/a><\/li>\n<li><a href=\"#ReplaceNULLLValues\">Replace NULLL Values<\/a><\/li>\n<\/ol>\n<p><a name=\"OperationsonNULLValues\"><\/a><\/p>\n<h3>How does SQL treat null values?<\/h3>\n<p>The null values create a <strong>problem<\/strong> while performing a relational operation that includes <strong>arithmetic operation<\/strong>, <strong>comparison operation<\/strong> and <strong>set operation<\/strong>.<\/p>\n<p><strong>1. Arithmetic Operations:<\/strong><\/p>\n<p>If any arithmetic expression including these (<strong>+, -, <sub>*<\/sub>,<\/strong> or<strong> \/<\/strong>) operators, have <strong>any of the input as null<\/strong>, will evaluate the result in <strong>null<\/strong>. For example, if we have to increment the salary of all the instructors by 100 $ then the expression would be:<\/p>\n<p style=\"text-align: center\"><strong><em>instructor.salary+100<\/em><\/strong><\/p>\n<p>In case, for a <strong>particular instructor<\/strong>, the <strong>salary value<\/strong> is <strong>null<\/strong> then the expression above will result in <strong>null<\/strong> for that particular instructor.<\/p>\n<p><strong>2. Comparison Operations:<\/strong><\/p>\n<p>The case is worse in comparison operations. Consider, if we have a comparison (1&lt;null). Now, how could you judge that the comparison will turn out to be true or it would be false? This is because we don\u2019t know what does null value exactly represents.<\/p>\n<p>So, it is very <strong>difficult<\/strong> to evaluate the result of a comparison that has a <strong>null value for comparison<\/strong>. Therefore, SQL evaluates the result of any comparison that includes null value, as <strong>unknown<\/strong>. SQL treats unknown as a <strong>logical value<\/strong> as it treats true and false.<\/p>\n<p>It may happen that the predicate in where clause includes <strong>Boolean operations<\/strong> to <strong>combine<\/strong> the results of <strong>comparisons<\/strong>. Therefore, the definition of Boolean operation must be extended in order to deal with the logical value &#8216;unknown&#8217;.<\/p>\n<p>Let us see how Boolean operations (and, or, and not) treat <strong>unknown<\/strong>:<\/p>\n<p><strong>and:<br \/>\n<\/strong>true <strong>and<\/strong> unknown = unknown<br \/>\nfalse <strong>and<\/strong> unknown = false<br \/>\nunknown <strong>and<\/strong> unknown = unknown<\/p>\n<p><strong>or:<br \/>\n<\/strong>true <strong>or<\/strong> unknown = true<br \/>\nfalse <strong>or<\/strong> unknown = unknown<br \/>\nunknown <strong>or<\/strong> unknown = unknown<\/p>\n<p><strong>not:<br \/>\n<\/strong><strong>not<\/strong> unknown = unknown<\/p>\n<p>In the predicate, the comparison between null i.e. <strong>null=null<\/strong> returns &#8216;unknown&#8217; instead of &#8216;true&#8217;. In case, the predicate in w<strong>here clause evaluates<\/strong> the result as <strong>false or unknown<\/strong> for any tuple in a relation. Then that <strong>tuple is not counted in the result.<\/strong><br \/>\n<a name=\"ISNULL&amp;ISNOTNULL\"><\/a><\/p>\n<h3>What is \u201cis null\u201d and \u201cis not null\u201d in SQL?<\/h3>\n<h4>Is Null Operator<\/h4>\n<p>In SQL, &#8216;is null&#8217; is used to <strong>test<\/strong> the predicate for the <strong>null value<\/strong>. For example, if we have to check for all those instructors who have their salary value equal to null in the instructor table below:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-773\" src=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Instructor-table.jpg\" alt=\"Instructor table\" width=\"520\" height=\"356\" srcset=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Instructor-table.jpg 520w, https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Instructor-table-300x205.jpg 300w\" sizes=\"auto, (max-width: 520px) 100vw, 520px\" \/><\/p>\n<p>The query for this would be:<\/p>\n<p style=\"text-align: center\"><em><strong>select<\/strong> name <strong>from<\/strong> instructor <strong>where<\/strong> salary <strong>is null;<\/strong><\/em><\/p>\n<p>As you can see in the image below this query will present the list of all the instructors who have a null value for the salary.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-771\" src=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/is-null.jpg\" alt=\"is null\" width=\"504\" height=\"134\" srcset=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/is-null.jpg 504w, https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/is-null-300x80.jpg 300w\" sizes=\"auto, (max-width: 504px) 100vw, 504px\" \/><\/p>\n<h4>Is Not Null Operator<\/h4>\n<p>Let us see the performance of &#8216;<strong>is not null<\/strong>&#8216; for the same query. Is not null is also used to <strong>test<\/strong> the <strong>predicate<\/strong> for the <strong>non-null values<\/strong>.<\/p>\n<p style=\"text-align: center\"><em><strong>select<\/strong> name <strong>from<\/strong> instructor <strong>where<\/strong> salary <strong>is not null;<\/strong><\/em><\/p>\n<p>Now, this query will list all the instructors who have non-null value for their salary.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-772\" src=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/is-not-null.jpg\" alt=\"is not null\" width=\"503\" height=\"270\" srcset=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/is-not-null.jpg 503w, https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/is-not-null-300x161.jpg 300w\" sizes=\"auto, (max-width: 503px) 100vw, 503px\" \/><\/p>\n<p><strong>3. Set Operations:<\/strong><\/p>\n<p>In the set operations, <strong>two tuples<\/strong> are treated as <strong>identical<\/strong>, if the <strong>values<\/strong> for corresponding <strong>attributes<\/strong> of<strong> both the tuples<\/strong> are either<strong> non-null<\/strong> and <strong>equal in value<\/strong> or if <strong>both are null<\/strong>.<\/p>\n<p>For example, we have two tuples:<\/p>\n<p><em>{(\u2018Steive\u2019, \u2018Biology\u2019, null), (\u2018Steive\u2019, \u2018Biology\u2019, null)}<\/em><\/p>\n<p>Both the tuples above are treated as identical as the <strong>first<\/strong> and <strong>second attribute values<\/strong>\u00a0of both the tuples are <strong>non-null and equal in value<\/strong> and the <strong>third attribute<\/strong> of both the tuple is <strong>null<\/strong>. They are considered identical though we don\u2019t know what does the null represent in both the tuples.<br \/>\n<a name=\"INSERTNULLValues\"><\/a><\/p>\n<h3>How to Insert NULL Values in SQL?<\/h3>\n<p>To Insert the NULL values into an attribute of an instance you just have to specify the NULL keyword into the VALUES clause of the INSERT statement Such as<\/p>\n<p>INSERT INTO instructor(instructor_id, name, department, salary) VALUES (164, Shovel, Physics, Null);<\/p>\n<p>This will insert an instructor instance with id 164, name Shovel, department Physics and his Salary attribute is assigned a NULL value.<br \/>\n<a name=\"ReplaceNULLLValues\"><\/a><\/p>\n<h3>How to Replace the NULL Values in SQL?<\/h3>\n<p>Well, you can always replace the NULL value of an attribute with a specific value using two SQL functions such as <strong>ISNULL()<\/strong> or <strong>COALESCE()<\/strong>. Let&#8217;s take the instructor table shown in the figure below has three NULL values for the attribute Salary.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-3135\" src=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Instructor-table-1.jpg\" alt=\"Instructor table 1\" width=\"446\" height=\"364\" srcset=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Instructor-table-1.jpg 446w, https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Instructor-table-1-300x245.jpg 300w\" sizes=\"auto, (max-width: 446px) 100vw, 446px\" \/><\/p>\n<p>Now if we want to replace the NULL value in the <em>Salary<\/em> column with the string &#8220;Not Decided&#8221; then we can do it using the ISNULL() function. Observe the command below:<\/p>\n<p><em>SELECT instructor_id, Name, Department\u00a0 \u00a0ISNULL(Salary, &#8216;Not Decided&#8217;) As Salary From Instructor;<\/em><\/p>\n<p>The command above will display the result as:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-3129\" src=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Result-of-ISNULL.jpg\" alt=\"Result of ISNULL\" width=\"501\" height=\"308\" srcset=\"https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Result-of-ISNULL.jpg 501w, https:\/\/binaryterms.com\/wp-content\/uploads\/2019\/11\/Result-of-ISNULL-300x184.jpg 300w\" sizes=\"auto, (max-width: 501px) 100vw, 501px\" \/><\/p>\n<p>For replacing the NULL with some specified label you can use the COALESCE similar to ISNULL such as:<\/p>\n<p><em>SELECT instructor_id, Name, Department\u00a0 \u00a0COALESCE(Salary, &#8216;Not Decided&#8217;) As Salary From Instructor;<\/em><\/p>\n<p>This query will produce the same result as above ISNULL. On the other hand, the COALESCE function returns the first non-null value from its arguments. For Example:<\/p>\n<p><em>SELECT COALESCE(NULL, NULL, Jenny, NULL, Biology);<\/em><\/p>\n<p><em>Output: Jenny<\/em><\/p>\n<p>So, this was all about the null values in SQL, we have seen the definition of null, how SQL treats problems raised due to null values. We have also seen the variation in results evaluated when the predicate is containing &#8216;is null&#8217; and &#8216;is not null&#8217;. We have also learned how to insert and replace the NULL values.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Null Value in SQL is the special value, allocated to an attribute in a relation that indicates, the information regarding the corresponding attribute is either unknown or doesn\u2019t even exist. In this section, we will discuss the null values in detail. What is the Null Value in SQL? The null value assigned to an attribute [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[4],"tags":[],"class_list":{"0":"post-770","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-dbms","7":"entry"},"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.6 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>What is Null Value in SQL? is null &amp; is not null in SQL - Binary Terms<\/title>\n<meta name=\"description\" content=\"Null Value in SQL is the special value, allocated to an attribute in a relation which indicates that the information regarding the corresponding attribute is either unknown or it doesn\u2019t even exist. 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