@Column(columnDefinition）的使用

columnDefinition属性的使用：

@Table(name = "CUSTOMERS")
@Entity
public class Customer {
    @Column(name = "ID")
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Id
    private Integer id;
    @Column(name = "Name")
    private String name;
    @Column(name = "Email",columnDefinition="varchar(128) not null")
    private String email;
    @Column(name = "Age")
    private int age;
    ......
}

数据库DDL：

CREATE TABLE `customers` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `Age` int(11) DEFAULT NULL,
  `Email` varchar(128) NOT NULL,
  `Name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

columnDefinition可以指定创建表时一些SQL语句，比如这里可以一次性指定，varchar长度128，且不能为空 
当然，相同的结果可以通过nullable和length属性结合实现

@Table(name = "CUSTOMERS")
@Entity
public class Customer {
    @Column(name = "ID")
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Id
    private Integer id;
    @Column(name = "Name")
    private String name;
    @Column(name = "Email", nullable = true, length = 128)
    private String email;
    @Column(name = "Age")
    private int age;
    ......
}

数据库DDL：

CREATE TABLE `customers` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `Age` int(11) DEFAULT NULL,
  `Email` varchar(128) DEFAULT NULL,
  `Name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

columnDefinition属性的特殊使用：
编程语言中字符串一般都用String表示，但是数据库中varcahr数值类型有长度限制，一旦需要大文本，则需要text数值类型
但是String类型默认映射的数值类型是varchar，columnDefinition可以进行额外指定

@Table(name = "CUSTOMERS")
@Entity
public class Customer {
    @Column(name = "ID")
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Id
    private Integer id;
    @Column(name = "Name")
    private String name;
    @Column(name = "Email", nullable = true, length = 128)
    private String email;
    @Column(name = "Age")
    private int age;
    @Column(name = "Remark",columnDefinition="text")
    private String remark;
    ......
}

数据库DDL：

CREATE TABLE `customers` (
  `ID` int(11) NOT NULL AUTO_INCREMENT,
  `Age` int(11) DEFAULT NULL,
  `Email` varchar(128) DEFAULT NULL,
  `Name` varchar(255) DEFAULT NULL,
  `Remark` text,
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

remark列数值类型是text，而Name和Email默认是varcahr

指定字段类型、长度、是否允许null、是否唯一、默认值

/** 仓库编号 */
@Column(name = "code",columnDefinition = "Varchar(100) not null default'' unique")
private String code;

需要特殊指定字段类型的情况

@Column(name = "remark",columnDefinition="text")
private String remark;

@Column(name = "salary", columnDefinition = "decimal(5,2)")
private BigDecimal salary;

@Column(name="birthday",columnDefinition="date")
private Date birthday;

@Column(name="createTime",columnDefinition="datetime")
private Date createTime;

@Column(name = "createTime", columnDefinition = "DATETIME DEFAULT CURRENT_TIMESTAMP ")
@Temporal(TemporalType.TIMESTAMP)
private Date createTime;

@Column(name = "updateTime", columnDefinition = "DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP")
@Temporal(TemporalType.TIMESTAMP)
private Date updateTime;