MySQL – Field Both NOT NULL and DEFAULT NULL

MySQLmysql-5.6null

This MySQL table had me perplexed for a moment:

mysql> desc quux;
+---------------------------+-----------------------+------+-----+---------+----------------+
| Field                     | Type                  | Null | Key | Default | Extra          |
+---------------------------+-----------------------+------+-----+---------+----------------+
| foobar                    | int(11)               | NO   |     | NULL    |                |
(...)

mysql> show create table quux;
(...)
`foobar` int(11) NOT NULL,
(...)

Since the foobar field was never created with a DEFAULT clause, it gets automatically assigned DEFAULT NULL. However, at a first sight this looks like contradicting the fact that it was also defined as NOT NULL.

Then I realized that this schema aims to force to insert a value (and a non-NULL one) in foobar when adding a new record.

Is this an acceptable way to do so, or there are better ways?

Best Answer

That's perfectly acceptable, it basically states:

You have to provide a value for this, but I have no clue what that would be.

If you define a default value, you claim that this is a sensible value for any row that doesn't supply a value - which is not really feasible for most attributes.

Think about a column named first_name - you want to make sure a value is supplied but there is no way you can come up with a sensible default value for that. Or a column named salary in an employee table.

Related Solutions

MySQL Default – NULL or NOT NULL?

In most DBs a NOT NULL column will be more efficient in terms of stored data for the reason you state, and also more efficient to query and index - so unless you want to allow NULLs in a column you should explicitly disallow them.

There will be a slight performance implication, as the extra NOT NULL constraints will potentially need to be checked for each row you affect with any INSERT or UPDATE but as most databases are relative write-light and read-heavy this is probably not a concern (the little extra time taken is unlikely to be noticeable at all anyway as it is a CPU-bound operation where the rest of the insert/update operation will be IO-bound and so a much more significant bottle-neck) and it gives you some "free" data checking so your code (or other people's code) can not accidentally put NULLs where other code is not expecting them and so may give incorrect results in their presence.

Edit: As Peter points out in his comment the above is a generalism and may not hold true for all DMBSs, though I'm pretty sure it does for mysql and mssql. Other complications in the area might include features like sparse tables (as implemented MSSQL 2008 for instance) which will alter the performance dynamics of (not)nullable columns.

MySQL – Get Column from Multiple Tables

If all the tables use the MyISAM Storage Engine and have the same table structure, I have some good news for you.

You can create a single table that consumes no additional space except a .frm file and some mapping info. The key is to take advantage of the MERGE (MRG_MyISAM) Storage Engine.

Here is how you can do this:

CREATE TABLE XMerge LIKE X1;
ALTER TABLE XMerge ENGINE=MRG_MYISAM
UNION=(X1,X2,X3,X4) INSERT_METHOD=LAST;

Using this method, you can query the 4 tables at the same time like this:

SELECT ReqF FROM XMerge WHERE EmpName='John';

Was that simple, or what ???

In your case, you have 75 tables. You would do this:

CREATE TABLE XMerge LIKE X1;
ALTER TABLE XMerge ENGINE=MRG_MYISAM
UNION=(X1,X2,X3,X4,X5,X6,X7,X8,X9,
X10,X11,X12,X13,X14,X15,X16,X17,X18,X19,
X20,X21,X22,X23,X24,X25,X26,X27,X28,X29,
X30,X31,X32,X33,X34,X35,X36,X37,X38,X39,
X40,X41,X42,X43,X44,X45,X46,X47,X48,X49,
X50,X51,X52,X53,X54,X55,X56,X57,X58,X59,
X60,X61,X62,X63,X64,X65,X66,X67,X68,X69,
X70,X71,X72,X73,X74,X75) INSERT_METHOD=LAST;
SELECT ReqF FROM XMerge WHERE EmpName='John';

The beauty of this is that creating a MERGE table takes milliseconds. Just make sure every table has an index on EmpName. Better to do 75 indexed lookups that 75 full table scans. If there is no index on EmpName, you need to do this:

ALTER TABLE X1 ADD UNIQUE KEY (EmpName);
ALTER TABLE X2 ADD UNIQUE KEY (EmpName);
.
.
.
ALTER TABLE X75 ADD UNIQUE KEY (EmpName);
CREATE TABLE XMerge LIKE X1;
ALTER TABLE XMerge ENGINE=MRG_MYISAM
UNION=(X1,X2,X3,X4,X5,X6,X7,X8,X9,
X10,X11,X12,X13,X14,X15,X16,X17,X18,X19,
X20,X21,X22,X23,X24,X25,X26,X27,X28,X29,
X30,X31,X32,X33,X34,X35,X36,X37,X38,X39,
X40,X41,X42,X43,X44,X45,X46,X47,X48,X49,
X50,X51,X52,X53,X54,X55,X56,X57,X58,X59,
X60,X61,X62,X63,X64,X65,X66,X67,X68,X69,
X70,X71,X72,X73,X74,X75) INSERT_METHOD=LAST;
SELECT ReqF FROM XMerge WHERE EmpName='John';

Give it a Try !!!

Best Answer

Related Solutions

MySQL Default – NULL or NOT NULL?

MySQL – Get Column from Multiple Tables

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