Handling NULL values
Operations containing NULL are NULL, except concatenation
|3*NULL+5 ||'HELLO' || ||NULL || ||'WORLD'|
|(null) ||Hello world || || || |
NVL2 to get a different result if a value is null or not
If the first parameter is NOT NULL, NVL2 will return the second parameter. Otherwise it will return the third one.
COALESCE to return the first non-NULL value
In some case, using COALESCE with two parameters can be faster than using NVL when the second parameter is not a constant. NVL will always evaluate both parameters. COALESCE will stop at the first non-NULL value it encounters. It means that if the first value is non-NULL, COALESCE will be faster.
Columns of any data type can contain NULLs
Empty strings are NULL
NVL to replace null value
NVL is useful to compare two values which can contain NULLs :
NULL can't appear in columns restricted by a PRIMARY KEY or a NOT NULL constraint.
(Exception is a new constraint with NOVALIDATE clause)