Can We Have NULL Value in Primary Key? - Interview Question of the Week #075 - SQL Authority with Pinal Dave (2023)

SQL Interview Questions and Answers

8 Comments

8 Comments. Leave new

• 

  Lokee

  June 12, 2016 9:10 am

  What about unique key. How unique key maintain only one null value.

  Reply
• 

  Lokee

  June 12, 2016 9:12 am

  What about unique key. How it maintains only one null values.

  Reply
• 

  Vinod Andani

  June 13, 2016 4:47 pm

  Pinal, “If two records of a single column have a NULL value, the column values are not considered equal”, then why GROUP BY considers NULLS as same when more than 1 record have nulls and groups by as same. Same case with distinct.
  Purpose of Primary Key cannot have Nulls : It should contain a valid atomic value which uniquely identifies record in the table, which can be indexed and further be linked to other tables using Foreign Keys. Nulls are UNKNOWN and unknowns cannot be referred nor is a candidate value and so Primary Key cannot have nulls.

  some examples :

  create table test_nulls (id int identity(1,1),
  col_value varchar(25))
  go

  insert into test_nulls(col_value)
  values(null), (‘NULL’), (‘some value’), (null),(’10’), (null),(‘text’)
  go
  select * from test_nulls

  select col_value, count(1) from test_nulls
  group by col_value

  Reply
• 

  harsh

  June 16, 2016 10:08 am

  Purpose of Primary Key cannot have Nulls : It should contain a valid atomic value which uniquely identifies record in the table, “which can be indexed and further be linked to other tables using Foreign Key. Nulls are UNKNOWN and unknowns cannot be referred …”
  Not agreed with this statement.

  To reference any field as foreign key, its not necessary that it should be primary key. Unique key can also referred as foreign key. Unique key can contain NULL values and so Foreign key can contain NULL values too.

  Reply
• 

  Vinod Andani

  July 18, 2016 7:11 pm

  Yes its true that Unique Key can also be referred as foreign keys, Thanks for letting me know this !! But you cannot make Unique keys as potential candidate unless imposing NOT NULL. And its of no value to represent NULL as a candidate value and so Primary Keys cannot have NULLs. On the other hand, attributes with Primary Keys when referred as foreign keys in child tables can have NULLs for which parent value is unknown.

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• 

  Yashveer Gurjar

  August 31, 2016 12:25 am

  Hi Pinal.As far as I Know.Primary key is a combination of NOt NULL and Unique .Then according to that concept it will not accept null values.

  In concern with UNIQUE constraint it will only accept 1 NULL value

  Reply
• 

  Megan Brooks

  August 31, 2016 9:22 pm

  A primary key is not only a unique identifier but also a value that can satisfy a predicate such as WHERE = , used to look up a unique row (assuming a simple PK). A NULL cannot directly substitute for a non-NULL value in that role, even if it is unique, because either a different predicate would be required depending on whether or not the key value was NULL or else a non-SARGable expression would have to be used in place of the PK column name in the predicate. NULL is just not on an equal footing with non-NULL values when used to uniquely identify rows.

  So maybe it is a useful interview question after all?

  Reply
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  Tom Lint

  August 20, 2021 2:22 pm

  I’ve never been able to understand why a SQL NULL doesn’t behave logically, like null in other programming languages. NULL is the absence of a value. There is nothing ‘unknown’ about it. And, as others have stated, SQL is terribly inconsistent with it, with a UNIQUE constraint only allowing a single NULL value, despite it being unequal to NULL, and therefore, according to the rules, never qualifying for uniqueness.

  Reply