检查数据库中的空值

Question

I have a database with stores a sizeable chunk of binary data. A remote app which checks the database in the office.

I basically want the remote app to check that the data exists without having to 'download' it from the remote database.

SELECT BinaryData FROM DB WHERE BinaryData IS NOT NULL

would work, but it would download the data also, I just want a simple check.

Any ideas

Answer 1

You can select the count of Not Null rows. This way you will not have to download the binary data to the remote app.

SELECT COUNT(*) FROM DB WHERE BinaryData IS NOT NULL

Answer 2

只是不要选择要下载的任何数据。

SELECT NULL FROM DB WHERE BinaryData IS NOT NULL

Answer 3

另一种检查非空值的方法

SELECT COUNT(*) as Count_BinaryData FROM DB WHERE DATALENGTH(BinaryData) > 0 

Answer 4

If you are using SQL Server (2008+) you can use EXISTS which could be faster than simple COUNT(*) because it stops selecting rows on first match.

IF EXISTS (SELECT 1 FROM DB WHERE BinaryData IS NOT NULL)
    SELECT 1
ELSE SELECT 0

Alternatively you can write something like this (in this case you will get 1 if there is a match and no rows selected if there isn't)

SELECT 1 WHERE EXISTS (SELECT 1 FROM DB WHERE BinaryData IS NOT NULL)

More examples can be found in docs .