Why Does \'SELECT DISTINCT\' Fail With Multiple Fields in MySQL?

Mysql Optimization: Understanding "SELECT DISTINCT" Limitations

In MYSQL, the "SELECT DISTINCT" clause is used to eliminate duplicate rows based on specified column values. However, it has certain restrictions that can be encountered when attempting to select multiple fields.

Problem:

When using "SELECT DISTINCT" followed by a comma-separated list of fields and another "DISTINCT" clause, as shown in the following example, an error occurs:

mysql_query("SELECT *, DISTINCT ticket_id FROM temp_tickets ORDER BY ticket_id")

Explanation:

Unlike a function that applies to individual columns, "DISTINCT" is a query modifier that affects all columns in the select-list. It removes duplicates only when all columns in the selection have identical values.

Therefore, the query attempts to output rows that are distinct based on all columns, including those specified in the comma-separated list and "ticket_id." This creates an ambiguity, as it's unclear which columns should be considered for uniqueness.

Solution:

To select all fields and eliminate duplicates based on "ticket_id," the correct syntax is:

SELECT DISTINCT *, ticket_id FROM temp_tickets ORDER BY ticket_id

This query will remove duplicates based on the combined values of all columns, including "ticket_id." However, it will still only show the latest entry for each unique "ticket_id" value. To retrieve only the latest entries for all columns, a subquery can be used to select the maximum ticket_id for each distinct "ticket_id" value, as follows:

SELECT * FROM temp_tickets WHERE ticket_id IN (SELECT MAX(ticket_id) FROM temp_tickets GROUP BY ticket_id)

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