| Firebird Docset → Firebird Database Docs → Firebird Null Guide → Summary |
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NULL in a nutshell:
NULL means unknown.
If NULL figures in an expression, most of the time the entire expression becomes NULL.
In aggregate functions only non-NULL fields are involved in the computation. Exception: COUNT(*).
UDFs sometimes convert NULL <-> non-NULL in a seemingly random manner.
If the test expression of an if statement is NULL, the then block is skipped and the else block executed.
To find out if A is NULL, use “A is (not) null”.
The COALESCE and *NVL functions can convert NULL to a value.
Assigning NULL is done like assigning values: with “A = NULL” or an insert list.
Remember, this is how NULL works in Firebird SQL. There may be (subtle) differences with other RDBMSes.
| Firebird Docset → Firebird Database Docs → Firebird Null Guide → Summary |