Why Does MySQL \'SELECT DISTINCT\' Cause Errors When Selecting All Fields?

MySQL "SELECT DISTINCT" Dilemma

In MySQL database management, "SELECT DISTINCT" is a query modifier used to eliminate duplicate rows from the result set. However, a common issue arises when trying to select all fields in a table along with the distinct values.

The query SELECT DISTINCT ticket_id FROM temp_tickets ORDER BY ticket_id effectively retrieves only the unique ticket_id values. However, when extending the query to SELECT * , DISTINCT ticket_id FROM temp_tickets ORDER BY ticket_id, an error occurs due to the incorrect placement of "DISTINCT."

The Correct Syntax

"DISTINCT" must be positioned immediately after "SELECT" in the query statement. Here is the correct syntax:

SELECT DISTINCT ticket_id, OTHER_COLUMN1, OTHER_COLUMN2, ... FROM temp_tickets ORDER BY ticket_id

In this syntax, "DISTINCT" applies to all columns listed in the select list, including the columns used for sorting (ORDER BY). Therefore, the query will return a distinct row for each unique combination of values in the listed columns.

Logical Explanation

"DISTINCT" eliminates duplicate rows if all columns in the select list have identical values. This means that if any other column contains a different value, the row will be considered unique and included in the result set.

In the incorrect syntax SELECT foo, DISTINCT ticket_id FROM table..., the query is ambiguous because it's unclear how to determine which rows should be eliminated. If there are three distinct values for ticket_id and six distinct values for foo, it's not clear which three values of foo should be output alongside the distinct ticket_id values. By placing "DISTINCT" immediately after "SELECT," the query becomes clear and eliminates any ambiguity.

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