What is the difference between SERIAL and AUTO_INCREMENT in mysql

Question

As per the docs

SERIAL is an alias for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.

So, be careful when creating a reference to a SERIAL PK, since that reference column has to be of this exact type.

AUTO_INCREMENT is an attribute of a specific column of any numeric type (int or float), both signed and unsigned. When rows are inserted it automatically assigns sequential numbers, so you don't have to (e.g. by using LAST_INSERT_ID()). See http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html

SERIAL is an alias that combines column type casting (BIGINT specifically), AUTO_INCREMENT, UNSIGNED and other attributes for a specific column (see quote from docs below). See https://dev.mysql.com/doc/refman/8.0/en/numeric-type-syntax.html

SERIAL is an alias for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.

SERIAL DEFAULT VALUE in the definition of an integer column is an alias for NOT NULL AUTO_INCREMENT UNIQUE.

From mysql doc

SERIAL is an alias for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.

SERIAL DEFAULT VALUE in the definition of an integer column is an alias for NOT NULL AUTO_INCREMENT UNIQUE.

If no value is specified for the AUTO_INCREMENT column, MySQL assigned sequence numbers automatically. You can also explicitly assign NULL or 0 to the column to generate sequence numbers. MySQL doesn't automatically decrease the autoincrement value when you delete a row. Reasons are:

• Danger of broken data integrity (imagine multiple users perform deletes or inserts...doubled entries may occur or worse)
• Errors may occur when you use master slave replication or transactions