同一参数中的空值和非空值

Question

我有这个：

Decalre @param int
select * from table where param = @param

有时人们想选择param为null的记录。

所以，像这样：

if @param is null
begin
select * from table where param is null
end
else select * from table where param =@param

是否可以将其合并到一个查询中？

Answer 1

应该能够在where子句中掩盖它：

select * from table where (param =@param) or (param is null and @param is null) 

Answer 2

使用coalesce替换null值，性能会比我认为的or更好：

SELECT * 
FROM table 
WHERE Coalesce(@param, 1) = Coalesce(param, 1)

Answer 3

select 
  * 
from table 
where (@param is null AND param is null) OR (@param is not null AND param = @param)