值的处理方案解决MSSQL中null值返回的有效方法（mssql 返回null）
There are times when the query of a table in a MSSQL database may return a null value. This can be a hassle, and can cause issues in many applications that rely on the query. In order to resolve this issue, some effective solutions can be employed. The following are a few solutions and strategies that can be employed to resolve the MSSQL null value return.
Firstly, the programmer should check for and understand the reason for the null value being returned. One of the most common causes of this is that the target table does not contain the necessary information. Ensuring that the target table contains the necessary information can help to avoid this issue.
Another solution is to use IFNULL/ISNULL functions. These functions can assign a value to null results so they don’t have to be manually handled. The syntax of the functions is fairly simple and can be used to assign a value to null rows quickly. Following is an example of the syntax of these functions:
SELECT x, IFNULL(y, ‘My New Value’)
FROM table_name
The programmer can also utilize stored procedures and cursors to handle null values. A stored procedure can be created for each table that retrieves all the required information and handles the null values. This allows the programmer to avoid manually coding in a solution for each query. The code for a cursor may look similar to following:
DECLARE @Var1 NVARCHAR(50)
DECLARE cur CURSOR FOR
SELECT Col1
FROM TableName
OPEN cur
FETCH NEXT FROM cur INTO @Var1
WHILE @@FETCH_STATUS = 0
BEGIN
IF @Var1 IS NULL
BEGIN
// do something here
END
FETCH NEXT FROM cur INTO @Var1
END
CLOSE cur
DEALLOCATE cur
The programmer may also reduce the amount of null values by using LEFT OUTER JOIN. Using this technique can often resolve the issue of a table having too many null values. The basic syntax of this is as follows:
SELECT a.Val1,b.Val2
FROM tableA a
LEFT OUTER JOIN tableB b
ON a.Val1 = b.Val2
Finally, the programmer can also use COALESCE and NULLIF functions. These functions are useful for managing the different types of data in a query. The following is an example of the syntax:
SELECT COALESCE(Val1, Val2, Val3)
FROM TableName
In conclusion, there are various solutions for handling null values when using MSSQL. Each of these solutions can be employed to resolving this common issue. By understanding the cause of the null values, using IFNULL/ISNULL functions, utilizing stored procedures/cursors, reducing null values with LEFT OUTER JOIN, and utilizing COALESCE and NULLIF functions, can often help overcome this issue.
