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Using the GROUP BY clause to group all rows by the target column(s) – i.e. the column(s) you want to check for duplicate values on. Using the COUNT function in the HAVING clause to check if any of the groups have more than 1 entry; those would be the duplicate values.
Duplicates in SQL are mostly the data points that exist more than once in our data store. For example: If we consider a customer table and in it, we store the details of customers of any particular shop. Then here, duplicate records would be the entry of a single customer more than once.
The go to solution for removing duplicate rows from your result sets is to include the distinct keyword in your select statement. It tells the query engine to remove duplicates to produce a result set in which every row is unique.
The WITH clause in SQL was introduced in standard SQL to simplify complex long queries, especially those with JOINs and subqueries. Often interchangeably called CTE or subquery refactoring, a WITH clause defines a temporary data set whose output is available to be referenced in subsequent queries.
The SQL WITH clause was introduced by Oracle in the Oracle 9i release 2 database. The SQL WITH clause allows you to give a sub-query block a name (a process also called sub-query refactoring), which can be referenced in several places within the main SQL query. The name assigned to the sub-query is treated as though it was an inline view or table. The SQL WITH clause is basically a drop-in replacement to the normal sub-query.
Syntax For The SQL WITH Clause
The following is the syntax of the SQL WITH clause when using a single sub-query alias.
WITH <alias_name> AS (sql_subquery_statement)
SELECT column_list FROM <alias_name>[,table_name]
[WHERE <join_condition>]
When using multiple sub-query aliases, the syntax is as follows.
WITH <alias_name_A> AS (sql_subquery_statement),
<alias_name_B> AS(sql_subquery_statement_from_alias_name_A
or sql_subquery_statement )
SELECT <column_list>
FROM <alias_name_A>, <alias_name_B> [,table_names]
[WHERE <join_condition>]
In the syntax documentation above, the occurrences of alias_name is a meaningful name you would give to the sub-query after the AS clause. Each sub-query should be separated with a comma Example for WITH statement. The rest of the queries follow the standard formats for simple and complex SQL SELECT queries.
For more information: http://www.brighthub.com/internet/web-development/articles/91893.aspx
This has been fully answered here.
See Oracle's docs on SELECT to see how subquery factoring works, and Mark's example:
WITH employee AS (SELECT * FROM Employees)
SELECT * FROM employee WHERE ID < 20
UNION ALL
SELECT * FROM employee WHERE Sex = 'M'
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