Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects simply to a non-technical, business audience. Over…
Null, in a database context, is the total absence of a value in a certain field and means that the field value is unknown. Null is not the same as a zero value for a numerical field, text field or space value. Null implies that a database field value has not been stored.
A null cannot be compared to a value. For example, if a query is directed to a Customer_Addresses table to retrieve all customers without email addresses, then the Structured Query Language (SQL) query cannot be written as follows: SELECT * FROM Customer_ Addresses WHERE Email_Address=null. Instead, so as not to introduce a comparison with a null, the query must be written as follows: SELECT * FROM Customer_ Addresses WHERE Email_Address IS null.
When the values in a column containing nulls are counted, nulls are not included in the results. For example, there are 200 customers in the Customer_ Addresses table, and 30 have nulls in the Email_Address column. Doing a count using the Email_Address column will return a result of 170.
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Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.
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