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An INSERT statement can also be used to retrieve data from other tables, modify it if necessary and insert it directly into the table. All this is done in a single SQL statement that does not involve any intermediary processing in the client application. A subselect is used instead of the VALUES clause. The subselect can contain joins, function ...
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 ...
As seen in the table above, direct equality comparisons between two NULLs in SQL (e.g. NULL = NULL) return a truth value of Unknown. This is in line with the interpretation that Null does not have a value (and is not a member of any data domain) but is rather a placeholder or "mark" for missing information.
The following table lists the standard-conforming values - based on SQL:2011. [1] The table's last column shows the part of the standard that defines the row. If it is empty, the definition originates from part 2 Foundation .
The old and new values are exposed as fields of a record data structures called :OLD and :NEW. It is necessary to test the nullity of the fields of the :NEW record that define the primary key (when a DELETE operation is performed), in order to avoid the insertion of a new row with null values in all columns.
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. [1]
The Null marker indicates the absence of a value, and is distinct from a value of 0 for an integer column or an empty string for a text column. The concept of Nulls enforces the 3-valued-logic in SQL , which is a concrete implementation of the general 3-valued logic .
Codd's twelve rules [1] are a set of thirteen rules (numbered zero to twelve) proposed by Edgar F. Codd, a pioneer of the relational model for databases, designed to define what is required from a database management system in order for it to be considered relational, i.e., a relational database management system (RDBMS).