Can I use a function for a default value in MySql?

No, you can't.

However, you could easily create a trigger to do this, such as:

CREATE TRIGGER before_insert_app_users
  BEFORE INSERT ON app_users 
  FOR EACH ROW
  SET new.api_key = uuid();

As of mysql v8.0.13 it is possible to use an expression as a default value for a field:

The default value specified in a DEFAULT clause can be a literal constant or an expression. With one exception, enclose expression default values within parentheses to distinguish them from literal constant default values.

CREATE TABLE t1 (
  uuid_field     VARCHAR(32) DEFAULT (uuid()),
  binary_uuid    BINARY(16)  DEFAULT (UUID_TO_BIN(UUID()))
);

As already stated you can't.

If you want to simulate this behavior you can use a trigger in this way:

CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
  IF new.uuid IS NULL
  THEN
    SET new.uuid = uuid();
  END IF;

You still have to update previously existing rows, like this:

UPDATE app_users SET uuid = (SELECT uuid());

Unfortunately no, MySQL 5 requires constants for the default. The issue was discussed in much more detail in the link below. But the only answer is to allow null and add a table trigger.

MySQL only recently accepted UUID as part of their DB package, and it's not as feature rich as we'd like.

http://www.phpbuilder.com/board/showthread.php?t=10349169

Can I use a function for a default value in MySql?

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