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Data cleaning: Difference between revisions
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** [http://www.mysqltutorial.org/mysql-distinct.aspx MySQL DISTINCT - Eliminate Duplicate Rows in a Result Set]. Using {{kbd | key =GROUP_CONCAT}} to handle the multiple columns<ref>[http://stackoverflow.com/questions/12188027/mysql-select-distinct-multiple-columns sql - MySQL SELECT DISTINCT multiple columns - Stack Overflow]</ref> | ** [http://www.mysqltutorial.org/mysql-distinct.aspx MySQL DISTINCT - Eliminate Duplicate Rows in a Result Set]. Using {{kbd | key =GROUP_CONCAT}} to handle the multiple columns<ref>[http://stackoverflow.com/questions/12188027/mysql-select-distinct-multiple-columns sql - MySQL SELECT DISTINCT multiple columns - Stack Overflow]</ref> | ||
** [http://www.w3schools.com/sql/sql_unique.asp SQL UNIQUE Constraint] "Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table." Quoted from w3schools webpage. | ** [http://www.w3schools.com/sql/sql_unique.asp SQL UNIQUE Constraint] "Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table." Quoted from w3schools webpage. | ||
** "{{kbd | key = UNION}} removes duplicates, whereas {{kbd | key = UNION ALL}} does not." source: [http://stackoverflow.com/questions/49925/what-is-the-difference-between-union-and-union-all sql - What is the difference between UNION and UNION ALL? - Stack Overflow] | |||
=== counting number of duplicate occurrence === | === counting number of duplicate occurrence === | ||