Why are NULL values needed in the relational model?
Null or NULL is a special marker used in Structured Query Language to indicate that a data value does not exist in the database. Codd, SQL Null serves to fulfil the requirement that all true relational database management systems (RDMS) support a representation of “missing information and inapplicable information”.
Can null values be avoided?
NULL value means that no entry has been made into the column. They should be avoided to avoid the complexity in select & update queries and also because columns which have constraints like primary or foreign key constraints cannot contain a NULL value.
How are nulls viewed in a relational database?
Nulls are negatively viewed from the perspective of database normalization. The idea being that if a value can be nothing, then you really should split that out into another sparse table such that you don’t require rows for items which have no value.
When do you need a null field in a database?
In some cases having a null field is useful, though, especially when you want to avoid yet another join for performance reasons (although this shouldn’t be an issue if the database engine is setup properly, except in extraordinary high performance scenarios.) One argument against nulls is that they don’t have a well-defined interpretation.
Which is an example of null values in a table?
As a trivial example, consider a customer account table with an end date as one of the columns – until the customer closes their account, the end date will be NULL. You are therefore likely to have a large number of NULL values in the end date column, yet it would be a bad choice to move this out into a separate table.
Are there null values in MS SQL Server?
When you set up a database (at least in MS SQL Server) you can flag a field as allowing NULL values and which default values to take. If you look through people’s DB structures, you’ll see that a lot of people allow NULL values in their database. This is a very bad idea.