I wrote this code today.
Copy Code code as follows:
DECLARE @atr NVARCHAR (20)
SET @atr = NULL
IF (@atr = NULL)
BEGIN
PRINT 1
End
Originally wanted to print out 1. But no. Modify the code as follows:
DECLARE @atr NVARCHAR (20)
SET @atr = NULL
IF (@atr is NULL)
BEGIN
PRINT 1
End
This will print out the correct 1.
Then you have made the following changes to the IF statement to modify the
Copy Code code as follows:
IF (null = NULL)
BEGIN
PRINT 1
End
Results: 1 is not printed
From this we can conclude that the null value in the SQL statement and any values are not equal when doing "=" operations. The judgment of whether to use ' is null ' to make a null value.
We can also use function ISNULL (@str, 0) = 0来 to determine whether a variable is a null value.
Small problem, record, strengthen memory, good at summarizing, do not walk long way.
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Added: NULL and "+" operators.
"+" can be used to do string concatenation, but also to do addition operations, then null value and string to do "+" what would be the result? I wrote the following validation program:
Copy Code code as follows:
DECLARE @str NVARCHAR (200)
DECLARE @str1 NVARCHAR (200)
DECLARE @str2 NVARCHAR (200)
SET @str = NULL
SET @str1 = ' look pretty '
SET @str2 = @str + @str1
PRINT @str2
As a result, nothing has been printed out, why? Let's test @str2 what's the value now?
IF (@str2 is NULL)
PRINT 1
The result prints out 1, so we can guess null and any type doing "+" result is still a null value
Then write a null and int type to do "+" validator:
Copy Code code as follows:
DECLARE @num INT
DECLARE @num1 INT
DECLARE @num2 INT
SET @num = 1
SET @num1 = 2
SET @num2 = @num + @num1
PRINT @num2
The result prints out 3, replacing the red bold part with the SET @num = NULL, and what is the result? Try it yourself, haha.
