MYSQL auto_increment, default keyword

1.

Auto_increment:

Only one column in the InnoDB table can be auto_increment, and this column must also be indexed.

CREATE TABLE T (X int auto_increment primary key,y int); --To

CREATE table T (x int auto_increment,y int, index ix_a (x))--the auto_increment can be indexed in the wrong way

CREATE TABLE T (X int auto_increment,y int auto_increment); --the wrong table can only have one auto_increment column

2.

Default

Its value is either constant, now (), or current_date ()

CREATE TABLE T3 (X int default 100,y datetime default Now ());

INSERT into T3 (x, Y) VALUES (default,default);

INSERT into T3 (x, Y) VALUES (null,null);

MYSQL auto_increment, default keyword