In MySQL, what is the difference between SERIAL and AUTO_INCRMENT?

In MySQL, both SERIAL and AUTO_INCRMENT are used to define a sequence as the default value of a field. But they are technically different.

All numeric data types except BIT and DECIMAL support the AUTO_INCRMENT attribute. There can be only one AUTO_INCRMENT field per table, and sequences generated by an AUTO_INCRMENT field in one table cannot be used in any other table.

This property requires a UNIQUE index on the field to ensure that the sequence has no duplicates. By default, the sequence starts at 1 and is incremented by 1 for each insertion.

Example

mysql> Create Table Student(Student_id INT PRIMARY KEY NOT NULL AUTO_INCREMENT, Name Varchar(20));
Query OK, 0 rows affected (0.18 sec)

The query above declares Student_id AUTO_INCRMENT.

mysql> Insert Into Student(Name) values('RAM'),('SHYAM');
Query OK, 2 rows affected (0.06 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> Select * from Student;
+------------+-------+
| Student_id | Name  |
+------------+-------+
|          1 | RAM   |
|          2 | SHYAM |
+------------+-------+
2 rows in set (0.00 sec)

mysql> Show Create Table Student\G
*************************** 1. row ***************************
      Table: Student
Create Table: CREATE TABLE `student` (
   `Student_id` int(11) NOT NULL AUTO_INCREMENT,
   `Name` varchar(20) DEFAULT NULL,
   PRIMARY KEY (`Student_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

On the other hand, SERIAL DEFAULT VALUE is short for NOT NULL AUTO_INCRMENT UNIQUE KEY. The SERIAL DEFAULT VALUE keyword is supported for integer numeric types such as TINYINT, SMALLINT, MEDIUMINT, INT, and BIGINT.

Example

mysql> Create Table Student_serial(Student_id SERIAL, Name VArchar(20));
Query OK, 0 rows affected (0.17 sec)

mysql> Insert into Student_serial(Name) values('RAM'),('SHYAM');
Query OK, 2 rows affected (0.12 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> Select * from Student_serial;
+------------+-------+
| Student_id | Name |
+------------+-------+
|          1 | RAM   |
|          2 | SHYAM |
+------------+-------+
2 rows in set (0.00 sec)

mysql> Show Create Table Student_serial\G
*************************** 1. row ***************************
      Table: Student_serial
Create Table: CREATE TABLE `student_serial` (
   `Student_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
   `Name` varchar(20) DEFAULT NULL,
   UNIQUE KEY `Student_id` (`Student_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

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