SQL- unique constraint :
Unique constraint is used to ensure that the values in the columns are distinct are not.It provides a guarantee for uniqueness in a column or set of columns.UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
SYNTAX :
UPDATE table_name SET column1=value, column2=value2,... WHERE some_column=some_value
Must Read:
You can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
Syntax -> set Unique KEY constraint on CREATE TABLE :
MySQL:
CREATE TABLE table_name (column1 datatype, column2 datatype, ....., UNIQUE(column1) );
SQL Server / Oracle / MS Access:
CREATE TABLE table_name (column1 datatype UNIQUE, column2 datatype, ..... );