SQL Server如何在列为null时设置默认值

Question

这是我在SQL server中的sql:

SELECT user_Name ,user_pass ,user_hometown FROM user_baseinfo   

有时列可以为null.如果是,我想用默认值替换它.

Answer 1

SELECT
    IsNull(user_Name, 'Unknown Name') AS user_Name,
    IsNull(user_pass, 'Unknown Pass') AS user_pass,
    IsNull(user_hometown, 'Unknown Hometown') AS user_hometown
FROM user_baseinfo

但请注意,它IsNull具有第二个参数类型的优先级.如果第一个参数中的列的类型短于默认值,则这可能会导致不希望的默认值切割.在这种情况下Coalesce是更好的选择.

下面是表示之间的差异的例子IsNull和Coalesce:

CREATE TABLE #Sample (Value1 varchar(100), Value2 char(3));

INSERT INTO #Sample (Value1, Value2)
VALUES
    ('AAA', 'AAA'),
    ('BBB', 'BBB'),
    ('CCC', 'CCC'),
    (NULL, NULL);

SELECT
    IsNull(Value1, 'Default') AS Value1,
    IsNull(Value2, 'Default') AS Value2
FROM #Sample;

SELECT
    Coalesce(Value1, 'Default') AS Value1,
    Coalesce(Value2, 'Default') AS Value2
FROM #Sample;

请参阅Default减少了到Def时IsNull被使用.

Answer 2

用例

select case 
          when user_Name is null then "default value" 
          else user_name 
       end  
from table