DOUBT

Question

IN SQL WHILE WE USING UNIQUE CONSTRAINT , ARE WE ALLOWED TO USE NULL VALUE OR NULL VALUES OR NONE OF THEM ????????

Comments

unique constraint use for defining that values of that field can not be duplicated.

it can have NULL also.

This is the major difference between unique key and candidate key.

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A unique constraint is a type of column restriction within a table, which dictates that all values in that column must be unique though may be null.

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so it can have one null only or many nulls in given column

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As per my knowledge
 In SQL, every null is different with others... Therefore it can have many null values

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SQL allows NULL by default. ( if you not apply constraint 'NOT NULL'.)

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As per this its possible. But it varies across specific different version of SQL.

Link: https://www.codeproject.com/Questions/488710/canplusweplussetplusuniquepluscolumnplusasplusnull

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ANSI SQL allows columns with UNIQUE constraints to have multiple NULLs. These NULLs don't count against the uniqueness of the column, because NULL is not equal to another NULL. 

Source: http://qr.ae/TUIddJ