Question

unique constraint in sql

Answer 1

The answer is unique constraint in Shankar quarrelling lining

Answer 2

• In SQL The UNIQUE constraint ensures the executed values in a column .In other words , no two rows can hold the same values for a common with UNIQUE constraint.

Let us take an example:—

CREATE TABLE statements:-

$\: \tt{CREATE \: TABLE \: Customer } \\ \tt{( \: \: \: \: SID \:integer \: \orange{Uni que}} \\ { \tt{Last\_na me\:var ch ar(30),}} \\ { \tt{First\_na me \:v ar ch ar(30) \: \: ) \: }}$

Column SID has a unique constraint, and hence cannot include duplicate values .Such constraint does not hold columns Last_Name and First_Name.So,it the table already contains the following rows:

$\begin{array}{c|c|c}\bf SID&\bf Last\_ Na me&\bf First\_ N a m e \\1&\sf Kumar&\sf Ayush\\2&\sf Chauhan&\sf Aditya\\3&\sf Ranjan& \sf Mukul\end{array}$

Excuting the following SQL commands

$\tt{INSERT\:INTO\:Customer} \: \: \: \: \: \: \: \: \: \: \: \\ \: \: \: \tt{VALUES('3','Cyrus','Grace')}$

It will result in an error because the value 3 already exist in SID column , thus trying to insert another row with that value violates the UNIQUE constraint.