0, "" (empty string), null, empty, difference between Nothing
First answer the following questions! After the following description, the variables A, B, C, D are equal to 0, respectively.
"", NULL, EMPTY, NOTING which one?
DIM A
DIM B AS STRING
DIM C AS Integer
DIM D As Object
A is equal to EMPTY because the "unstamined variable" that has not been initialized is equal to EMPTY. But if you check
TRUE values can also be obtained by testing A = "" or A = 0.
B is equal to "", because the non-fixed length "word」 "that has not been initialized is equal to" "". But please pay attention
B <> NULL.
C is equal to 0, is this still problem?
D is equal to Nothing, the "object variable" that has not been set is equal to Nothing, but please
To use D = Nothing, use D is nothing to determine if D is equal to Nothing,
Because the symbols that determine if the equal symbol is Is not =.
The most confusing place is Null this reserved word, please see the following statement:
Print x = NULL
Print x <> null
The result is the output null (not true is not false), this is because any of the arithmetic
To contain null, the arithmetic format is equal to NULL, in fact, if you want to determine if a data is null
Never use:
IF x = null life 'will always get null
To use:
IF isnull (x) THEN
Which data will be equal to NULL? In addition to the NULL operation, it is not input.
"Data Field" (in the database) of the data is equal to NULL.
