-- add unique constraint to multiple columnsALTERTABLECollegesADDUNIQUEUnique_College (college_id, college_code); Here, the SQL command adds theUNIQUEconstraint tocollege_idandcollege_codecolumns in the existingCollegestable. Also,Unique_Collegeis a name given to theUNIQUEconstraint defined forcollege_...
To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersonsADDUNIQUE(P_Id) To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use ...
What does the SQLUNIQUEconstraint ensure? That a column cannot contain NULL values That all values in a column are different That a column must always contain a value That a column is a foreign key ❮ PreviousNext ❯ Track your progress - it's free!
Applies to: SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed InstanceYou can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary...
1、使用sql查询主键的唯一约束当前最大值：select max（id） from table；（id为number类型）select max（to_number(id)） from table;(id为非number类型)2、使用sql查询该表的序列的下一个值 select SEQ.NEXTVAL from dual;（seq是对应表的序列名）3、最后使用PL/SQL客户端编辑序列，将序列的下一...
一、UNIQUE CONSTRAINT IN DATABASES 在数据库中,约束条件是用来限制所插入数据库中的数据类型、数量以及数据之间的关系等。其中,unique约束是一种常见的约束类型,它主要用于保证一个表中的一列或者是多列的组合值是唯一的。一旦这种约束被定义,任何试图插入重复值的操作都会被数据库系统拒绝。这种功能在处理那些需要唯...
test=#altertabletbl_uniqueaddconstraintuk_tbl_unique_a_bunique(a,b);ALTERTABLE 如果表中没有主键或NOT NULL的唯一键,那么可以利用表的OID属性,将表的oid列显示出来,该列类似主键的功能。利用该列,可以将重复数据删除到只剩一条,先使用下面的SQL语句,修改表的属性。
,CONSTRAINT emp_email_uk UNIQUE (email) ... ); Theemp_email_uk constraint ensures that no two employees have the same emailaddress, as shown in Example5-1. Example 5-1 Unique Constraint SQL> SELECT employee_id, last_name,email FROM employees WHERE email = 'PFAY'; ...
constraint fk_info1_depart1 foreign key (depart_id) references depart1(id) ) default charset=utf8; -- 如果表结构创建好了,额外添加外键 alter table info add constraint fk_info_depart foreign key info(depart_id) references depart(id);
3 rows in set mysql> 在修改表时创建唯一约束 在修改表时添加唯一约束的语法格式为: ALTER TABLE <数据表名> ADD CONSTRAINT <唯一约束名> UNIQUE(<列名>); 【实例3】修改学生表student,指定学生的名子唯一,输入的 SQL 语句和运行结果如下所示。