NULL is a unique data type in the database description of NULL in Oracle
NULL
Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data. When a column of a record is NULL, it indicates that the value of the row is unknown and is indeterminate, since it is unknown, there are countless possibilities. Therefore, NULL is not a deterministic value--judging a column as empty select * from Customeren C WHERE C.modifydt is null;--judging a non-empty select * from Customeren C wher E C.modifydt is not NULL; Test: --null=null: null is not equal to Nullselect case when null = NULL then & nbsp ' null is equal to NULL ' ELSE ' null is isn't equal to NULL ' end "Null=null?" from dual; --empty string =null: empty string is isn't equal to Nullselect case when "= NULL Then ' empty strings are equal to NULL ' ELSE ' empty string is not equal to NULL ' end "empty string =null?" from dual; --space =null: space is not equal to Nullselect case when ' = NULL the n ' spaces are equal to NULL ' ELSE ' space is not equal to NULL ' end "space =null?" from dual; --"is null:" Is NullseleCT case when "is null then " ' Is null ' &NBSP ; ELSE ' are not NULL ' end "' Is null?" from dual; --empty string = empty string: ' is not equal to ' SELECT case when ' = ' then ' is equal to ' ' ELSE &NBSP ; "is not equal to" " end" "" "" "?" from dual; --replace null with a value select 1 from DUAL where NVL (null,0) =NVL (null,0); Null sort problem Method 1 replaces null with a value that is used with the NVL function select 1 from dual where NVL (null,0) =NVL (null,0); Method 2 uses NULLS Last,nulls first to process the SELECT C.modifydt from the Customeren C ORDER by C.modifydt NULLS last; SELECT C.modifydt from Customeren C ORDER by C.modifydt NULLS first;
"ORACLE" special null
