来试试这道 MySQL 面试题吧，比 Leecode 好玩多了~~~

mysql> show create table x\G 
***********1.row*************
       Table:x
Create Table: CREATE TABLE `x`(
`a` INT NOT NULL AUTO INCREMENT,
`b` VARCHAR(1),
PRIMARY KEY (`a`)) ENGINE=InnODB;


mysql> SELECT * FROM x;
+---+------+
| a | b    |
+---+------+
| 0 | a    |
| 2 | b    |
+---+------+


mysql> ALTER TABLE x MODIFY COLUMN b VARCHAR(5);
复制

mysql> show create table y\G 
***********1.row*************
       Table:y
Create Table: CREATE TABLE `y`(
`a` INT,
`b` VARCHAR(1),
PRIMARY KEY (`a`)) ENGINE=InnODB;


mysql> SELECT * FROM y;
+---+------+
| a | b    |
+---+------+
| 0 | a    |
| 2 | b    |
+---+------+


mysql> ALTER TABLE y MODIFY COLUMN a INT AUTO_INCREMENT;
复制