how to make column unique in mysql code example

Example 1: mysql query unique column

SELECT DISTINCT(column_name) FROM your_table_name;

#You can select distinct values for one or more columns. The column names has to be separated with comma.
SELECT DISTINCT column_name_1, column_name_2 FROM your_table_name;

Example 2: mysql set field unique

ALTER TABLE mytbl ADD UNIQUE (columnName);

Example 3: mysql alter table set column unique

ALTER TABLE contacts
ADD CONSTRAINT contacts_unique UNIQUE (reference_number);