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In SQL, null or NULL is a special marker used to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd , SQL null serves to fulfill the requirement that all true relational database management systems ( RDBMS ) support a representation of "missing information and ...
Additionally, because SQL operators return Unknown when comparing anything with Null directly, SQL provides two Null-specific comparison predicates: IS NULL and IS NOT NULL test whether data is or is not Null. [8] SQL does not explicitly support universal quantification, and must work it out as a negated existential quantification.
A NOT NULL constraint is functionally equivalent to the following check constraint with an IS NOT NULL predicate: . CHECK (column IS NOT NULL) Some relational database management systems are able to optimize performance when the NOT NULL constraint syntax is used as opposed to the CHECK constraint syntax given above.
The SQL:1999 standard introduced a BOOLEAN data type as an optional feature (T031). When restricted by a NOT NULL constraint, a SQL BOOLEAN behaves like Booleans in other languages, which can store only TRUE and FALSE values. However, if it is nullable, which is the default like all other SQL data types, it can have the special null value also.
It is possible to put UNIQUE constraints on nullable columns but the SQL standard states that the constraint does not guarantee uniqueness of nullable columns (uniqueness is not enforced for rows where any of the columns contains a null). According to the SQL [2] standard a unique constraint does not enforce uniqueness in the presence of nulls ...
Meanwhile, COALESCE simplifies the process of handling NULL values by returning the first non-NULL value in a given list of expressions, which is especially useful in scenarios where data might be incomplete or missing. Furthermore, SQL's support for three-valued logic (True, False, Unknown) introduces nuances when handling NULL values in ...
CREATE TABLE employees (id INTEGER PRIMARY KEY, first_name VARCHAR (50) not null, last_name VARCHAR (75) not null, mid_name VARCHAR (50) not null, dateofbirth DATE not null); Some forms of CREATE TABLE DDL may incorporate DML ( data manipulation language )-like constructs, such as the CREATE TABLE AS SELECT (CTaS) syntax of SQL.
This function determines whether the expression passed as its argument evaluates to Null. A null value in any sub-expression of the expression causes the entire expression to be considered null. [15] IsNull(expression1,expression2) This function, taking two arguments, is specific to Transact-SQL.