在 MySql 列中接受空值，但默认情况下不接受

Question

I just can't can find an answer for my question. If there is on stack overflow, just post it and I will delete my post.

My problem:

I want to accept null values in a mysql column, but not by default. When I insert a row I should also set a value for that column and that should be required.

Right now, in phpmyadmin, if I check Null field (to accept null values), the field Default (default value) will be automatically set with null value. That means that I can omit to set a value for that column in an insert query and it will be filled automatically with a null value.

When did I say a column should be auto-filled if is not set ? I said to accept null values, not to be auto-filled.

Do I miss something ?

Answer 1

If a column has the NULL attribute, and you don't specify an explicit DEFAULT attribute, it's as if you defined it with DEFAULT NULL . This is specified in the documentation of"Implicit Default Handling" :

If the column can take NULL as a value, the column is defined with an explicit DEFAULT NULL clause.

If you want to prevent omitting the column when inserting, I think you can do it by specifying a DEFAULT expression that causes an error. This requires you to be using at least version 8.0.13 of MySQL, because prior to this the DEFAULT had to be a literal, not an expression.

Answer 2

Quotingthe docs :

Implicit Default Handling

If a data type specification includes no explicit DEFAULT value, MySQL determines the default value as follows: If the column can take NULL as a value, the column is defined with an explicit DEFAULT NULL clause.

So if your column is nullable, it always has a default value - whether set explicitly (by DEFAULT ... clause) or implicitly (like described above).

As @Barmar noticed, there's (at least in theory) a way to handle this with providing an always-failing DEFAULT expression (since MySQL 8.0.13 allowing it).

An alternative (available on versions before 8.0.13) is setting up a DEFAULT value that never should be put in your column anyway AND creating an pre-insert trigger checking the inserted value against this special case.

The benefit of this approach is you're more flexible in providing specific errors for that case. The downside is the value you might consider special will end up being actually used by real-world scenarios, causing a really weird bug. That's easier to prevent for numbers (0 for unsigned IDs, etc.), but much harder to do with user-created strings and similar entities.

Answer 3

You can do this by creating a column that accepts a null, and then applying a trigger BEFORE INSERT that checks for null.

For example:

CREATE TABLE `testNulls` (
  `id` int NOT NULL AUTO_INCREMENT,
  `nullField` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `id_UNIQUE` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 ;

Now create a trigger

DELIMITER $$
CREATE TRIGGER `testNulls_BEFORE_INSERT` BEFORE INSERT ON `testNulls` FOR EACH ROW BEGIN
  if (new.nullField is null) then  
      signal sqlstate '45000' set message_text = 'Cannot insert NULL in nullField';
      END IF;
END$$
DELIMITER ;

Attemppting to insert a NULL in the nullField column will return an error 45000, but updating a row to contain NULL will succeed.

Tested with MySQL 8 Demo: https://www.db-fiddle.com/f/8eV4VihbeB5woEYwuGMFvK/0