SQLServer field is determined, whether the variable is NULL or empty characters

First, the field is determined

(1) is empty

select * from department where leader is NULL
select * from department where leader is not NULL

(2) whether the null character ''

select * from department where leader=''

select * from department where leader!=''
select * from department where leader<>''

 

Second, determine the variables

(1) is empty

declare @r varchar(50)='abc'
	if @r is null
	begin
	  set @r='暂无数据'
	end 
	else if @r is not null
	begin
	  set @r='数据为：'+ @r
	end
select @r

------------- begin 和 end 不写也是阔以的，就是写上会显得规范些---------------

declare @r varchar(50)=NULL
	if @r is null
	  set @r='暂无数据'
	else if @r is not null
	  set @r='数据为：'+ @r
select @r

(2) whether the null character ''

declare @r varchar(50)='324'
	if @r = ''
	  set @r='暂无数据'
	else if @r != ''
	  set @r='数据为：'+ @r
select @r

--------------------------------

declare @r varchar(50)='bbb'
	if LEN(@r) = 0
	  set @r='暂无数据'
	else if LEN(@r) != 0
	  set @r='数据为：'+ @r
select @r