Collation sequences only apply to text. NULL is not valid text. A "null string" (that is, a string that does not contain anything) is actually a string and not NULL. So it really matters what you mean by NULL. A "null string" is a string (text) that does not contain anything. It is a string/text value, and that value is "empty" of content. Conversely NULL is a specific value that is of the NULL type. Collating sequences only apply to TEXT values.