Oracle/MySQL/SQL Server修改表字段类型和长度

ALTER TABLE T_USER MODIFY USERNAME VARCHAR2(18);

ALTER TABLE T_USER CHANGE USERNAME USERNAME VARCHAR(200) NOT NULL COMMENT ‘username‘;

ALTER TABLE T_USER ALTER COLUMN USERNAME VARCHAR(200);

1 ALTER TABLE T_USER DROP CONSTRAINT PK_T_USER;
2 ALTER TABLE T_USER ALTER COLUMN ID VARCHAR(20) NOT NULL;
3 ALTER TABLE T_USER ADD CONSTRAINT PK_T_USER PRIMARY KEY (ID);

1 ALTER TABLE T_USER DROP CONSTRAINT UN_T_USER;
2 ALTER TABLE T_USER ALTER COLUMN USERNAME VARCHAR(20);
3 ALTER TABLE T_USER ADD CONSTRAINT UN_T_USER UNIQUE (USERNAME);

1 DROP INDEX IDX_USER ON T_USER;
2 ALTER TABLE T_USER ALTER COLUMN USERNAME VARCHAR(20);
3 CREATE INDEX IDX_USER ON T_USER(USERNAME);