只返回空字段的行？

Question

我有一个表,其中一个字段可以为空.我试图只返回这个字段为空的行.但是我一直在做WHERE field =""...... WHERE field =''...... WHERE field = null

我缺少什么想法？

Answer 1

In SQL, a completely empty field is said to be NULL. For NULL searches you do not use equals (=), but rather the special operator IS NULL:

 SELECT * FROM table WHERE field IS NULL

Access allows you to have not NULL empty fields if you allow empty strings, this is a bad idea as it makes it difficult to distinguish visual between a NULL value and a 0-length string, so I suggest you don't permit it in your database.