How do i assign unique identifier for a record in employee table?

Question

I know it's first std question but someone asked me, but i was unable give him apropriate answer. There are two tables Employee,Technology having many to many relation saved in employee_technology_rel .Employee table having fields- emp_id (auto_increment) , emp_name(varchar) , DOB (date) where Technology having fields tech_id(auto_increment) , tech_name(varchar) these two tables allows duplication of names.which unique constraint should i define for allowing unique entry?

Answer 1

You can define the unique entry on table employee_technology_rel ,

ALTER TABLE employee_technology_rel 
       ADD CONSTRAINT emptech_pk PRIMARY KEY (emp_id, tech_id)
// or if you have set a primary key already, you can still define via UNIQUE
ALTER TABLE employee_technology_rel 
       ADD CONSTRAINT emptech_uq UNIQUE (emp_id, tech_id)

what it does is it only allows unique technology for every employee .

in order for you to have unique emp_name on table Employee as well as unique tech_name on table Technology , you can also alter the table by adding unique constraint

ALTER TABLE Employee ADD CONSTRAINT emp_uq UNIQUE (emp_name)
ALTER TABLE Technology ADD CONSTRAINT tech_uq UNIQUE (tech_name)

Answer 2

您需要在employee_technology_rel的两列上定义一个组合主键：emp_id和tech_id。

Answer 3

Unique Index and Unique Constraint are the same. They achieve same goal. SQL Performance is same for both.

Add Unique Constraint ALTER TABLE dbo. ADD CONSTRAINT UNIQUE NONCLUSTERED ( ) ON [PRIMARY]

Add Unique Index CREATE UNIQUE NONCLUSTERED INDEX ON dbo. ( ) ON [PRIMARY]

Source sqlauthority.com and msdn from Google search: " SQL server unique index ".