Whether the SQL statement query is empty =null and Null_mssql

A B c D

980515 Jing Top Enterprise Co., Ltd. simple home Hao NULL

980514 full Hin Wood Industry Co., Ltd. NULL 123

980514 Xun billion Technology Co., Ltd. Simple and NULL

980515 San Yue International Enterprise Co., Ltd NULL 111

Table structure as shown above, the SQL statement to query for a record in the C column is not a select * from table where c=null, or a select * from table where c= '; and should be a select * from table where c is null; On the contrary, the query is not empty should be select * from Talbe where c<> '; or select * from table where C was not null; Note: not ' is ' null OH.