TSQL字符串连接在WHILE循环中不起作用？

Code bellow is not working, any ideas why?

代码吼叫不起作用,任何想法为什么?

declare @Counter int
set @Counter = 0
declare @ConcText nvarchar(1000)

while @Counter < 5
begin
    --set @ConcText = @ConcText + cast(@Counter as nvarchar(10)) + N' counter,'
    --set @ConcText = @ConcText + convert(nvarchar(10), @Counter) + N' counter,'
    set @ConcText = @ConcText + N' counter,'
    set @Counter = @Counter + 1
end
print @ConcText --<-- this is null, why  ??

2 个解决方案

#1

See MSDN: + (String Concatenation) (Transact-SQL):

请参阅MSDN:+(字符串连接)(Transact-SQL):

Just like arithmetic operations that are performed on null values, when a null value is added to a known value the result is typically an unknown value, a string concatenation operation that is performed with a null value should also produce a null result.

就像对空值执行的算术运算一样,当将空值添加到已知值时,结果通常是未知值,使用空值执行的字符串连接操作也应该产生空结果。

So to get things work, it's a good practice to initiate varchar variables immediatly after declare:

因此,为了使事情有效,在声明后立即启动varchar变量是一个好习惯:

DECLARE @ConcText NVARCHAR(1000)
SET @ConcText  = ''

Other way to handle NULL concat issue (in case you don't know if value is NULL or not) - ISNULL or COALESCE:

处理NULL concat问题的其他方法(如果你不知道值是否为NULL) - ISNULL或COALESCE:

SET @ConcText = ISNULL(@ConcText, '') + N' counter,'
SET @ConcText = COALESCE(@ConcText, '') + N' counter,'

#2

You are not setting @ConcText to anything at the start, therefore when you concatenate anything to NULL you get NULL.

您没有在开始时将@ConcText设置为任何内容,因此当您将任何内容连接到NULL时,您将获得NULL。

#1