MySQL select default value - mysql

Question

I have foreign key in one table, references on another table, not null. How can I select the default value for it?
Something like this:
ALTER TABLE table_a MODIFY COLUMN not_null_column BIGINT NOT NULL DEFAULT
(SELECT id FROM table_b WHERE name_field = 'some name');
Or this:
SET #defaultValue = (SELECT id FROM table_b WHERE name_field = 'some name');
ALTER TABLE table_a MODIFY COLUMN not_null_column BIGINT NOT NULL DEFAULT #defaultValue;

Answer 1

One option is to use 13.5 Prepared SQL Statement Syntax.
mysql> DROP TABLE IF EXISTS `table_b`, `table_a`;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE IF NOT EXISTS `table_a` (
-> `not_null_column` VARCHAR(20)
-> );
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE IF NOT EXISTS `table_b` (
-> `name_field` VARCHAR(255) NOT NULL,
-> `value` VARCHAR(255) NOT NULL
-> );
Query OK, 0 rows affected (0.00 sec)
mysql> INSERT INTO `table_b`
-> (`name_field`, `value`)
-> VALUES
-> ('some name', '5');
Query OK, 1 row affected (0.00 sec)
mysql> DESC `table_a`;
+-----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| not_null_column | varchar(20) | YES | | NULL | |
+-----------------+-------------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> SET #`stmt` := CONCAT('ALTER TABLE `table_a`
'> MODIFY COLUMN `not_null_column` BIGINT NOT NULL
'> DEFAULT ', (SELECT `value`
-> FROM `table_b`
-> WHERE `name_field` = 'some name'));
Query OK, 0 rows affected (0.01 sec)
mysql> SELECT #`stmt`;
+-------------------------------------------------------------------------------------------------------------------------------+
| #`stmt` |
+-------------------------------------------------------------------------------------------------------------------------------+
| ALTER TABLE `table_a`
MODIFY COLUMN `not_null_column` BIGINT NOT NULL
DEFAULT 5 |
+-------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> PREPARE `stmt` FROM #`stmt`;
Query OK, 0 rows affected (0.00 sec)
Statement prepared
mysql> EXECUTE `stmt`;
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DEALLOCATE PREPARE `stmt`;
Query OK, 0 rows affected (0.00 sec)
mysql> DESC `table_a`;
+-----------------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+------------+------+-----+---------+-------+
| not_null_column | bigint(20) | NO | | 5 | |
+-----------------+------------+------+-----+---------+-------+
1 row in set (0.00 sec)
Example db-fiddle.