Thanks for the question, Seonghyun.
Asked: February 19, 2019 - 1:11 am UTC
Last updated: February 19, 2019 - 11:23 am UTC
Version: 11gR2 Express Edition for Windows x64
Viewed 1000+ times
You Asked
I asked you one last time and I will ask you one more question.
If !=0, we know we should include null, but the result is not null.
If !=0, it is different from zero and null, should not it also contain null if it is not 0?
and Chris said...
If !=0, we know we should include null, but the result is not null.No!
NULL = ANYTHING => UNKNNOWN
But also:
NULL != ANYTHING => UNKNNOWN
So
NULL != 0
returns
UNKNOWN
Where clauses only return rows where the conditions are TRUE
UNKNOWN <> TRUE
So any row that contains null in comm will NOT return if you search for:
comm != 0
Read up on three-valued logic:
https://en.wikipedia.org/wiki/Three-valued_logic
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