What's the point of “NOT NULL DEFAULT '' ”?

Question

I\'ve seen this on a lot of fields on a DB from a project I\'ve been working on, where a column will be defined not null, but will have an empty string as the default. what\'s t

Answer 1

NULLs have special behavior: comparing anything with a NULL gives you back a NULL, which is something else than false or 0. It means "unknown".

For example, take this table:

 user_id | gender
------------------
 1       | NULL
 2       | 'M'
 3       | 'F'
 4       | 'F'

SELECT * FROM mytable WHERE gender = 'M' will return 1 row, as expected

SELECT * FROM mytable WHERE gender != 'M' will return 2 rows, NOT 3 rows.

SELECT * FROM mytable WHERE gender != 'M' OR gender IS NULL will return the expected 3 rows.

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Edit: For some applications, using 0 (or, God forbid, another "magic number") instead of NULL is not even advisable (units or exact values are not relevant in this example):

 Date       | Temperature 
--------------------------
 2010-01-01 | 10          
 2010-01-02 | 4
 2010-01-03 | 0
 2010-01-04 | -22
 2010-01-05 | -45
 2010-01-06 | NULL
 2010-01-07 | -34

Here, the NULL on Jan 6th means "value unknown" - maybe because the temperature was so low that the thermometer probe stopped responding. However, it's a completely different meaning than Jan 3rd, when the temperature was 0, that is, 0 degrees.

Also, as @Bill Karwin mentions, NULLs behave specially in aggregate functions (COUNT,SUM,AVG etc.): calculating AVG(Temperature) on the above data would give you -14.5, as the NULL row is ignored.