UNIQUE Constraint With Alter Table We can also add theUNIQUEconstraint to an existing column using theALTER TABLEcommand. For example, For a Single Column -- add unique constraint to an existing columnALTERTABLE
The following article provides an outline for SQL UNIQUE Constraint. A single field or group of attributes that uniquely identify a record is referred to as a unique constraint. As long as the collection of values is distinct, some of the fields can have null values. Its purpose is to make...
IN ORACLE / SQL Server / MS Access: Syntax: ALTER TABLEDROP CONSTRAINT<constraint_name>; Example: ALTER TABLE STUDENTS DROP CONSTRAINT stu_Info; NOT NULL Constraint in SQL Introduction to SQL SQL – Combining AND, OR and NOT together in where clause SQL SELECT Database – USE Statement SQL ...
SQL > Constraint > UNIQUE Constraint The UNIQUE constraint ensures that all values in a column are distinct. For example, in the following CREATE TABLE statement, CREATE TABLE Customer (SID integer UNIQUE, Last_Name varchar (30), First_Name varchar(30));column "SID" has a UNIQUE ...
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!
SQL UNIQUE Key Constraint : 1 . At The Time Of Table Creation : UNIQUE Key Constraint is used to identify every record uniquely. Multiple Unique Key can be in a sigle table. SQL – UNIQUE Key Constraint Syntax Example : Example CREATE TABLE UserProfile ( ...
Using unique constraint in SQL To create a unique constraint, In Object Explorer, connect to an instance of Database Engine. On the Standard bar, click New Query. Copy and paste the following example into the query window and click Execute. This example creates the table SampleDetails and cre...
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...
CONSTRAINT emp_email_nn NOT NULL ... ,CONSTRAINT emp_email_uk UNIQUE (email) ... ); Theemp_email_uk constraint ensures that no two employees have the same emailaddress, as shown inExample5-1. Example 5-1 Unique Constraint SQL> SELECT employee_id, last_name,email FROM employees WHERE ...
SQL UNIQUE Constraint - Learn about the SQL UNIQUE constraint, its purpose, and how to implement it effectively in your database to ensure data integrity.