How does DISTINCT function in MySQL when selecting all columns?

Subtlety of DISTINCT in MySQL

While using MySQL's DISTINCT keyword to retrieve unique rows, users may encounter limitations when attempting to select all fields. To understand this behavior, let's delve into the workings of DISTINCT.

DISTINCT eliminates duplicate rows based on the uniqueness of the specified column. However, it cannot be selectively applied to only certain columns. Instead, it modifies the entire query, ensuring that all columns in the select-list are unique.

For example, the query:

<code class="mysql">SELECT DISTINCT ticket_id FROM temp_tickets ORDER BY ticket_id</code>

will successfully retrieve distinct values for the ticket_id column, excluding duplicate rows. However, the query:

<code class="mysql">SELECT * , DISTINCT ticket_id FROM temp_tickets ORDER BY ticket_id</code>

will result in an error because DISTINCT must directly follow SELECT and cannot be placed after column names.

To achieve the desired outcome of selecting all fields while ensuring that duplicate rows are eliminated, the correct syntax is:

<code class="mysql">SELECT DISTINCT * FROM temp_tickets ORDER BY ticket_id</code>

This query will return the distinct rows based on all columns in the table, effectively eliminating duplicates.

Understanding this subtle difference in DISTINCT's usage is crucial to avoid errors and correctly retrieve data from MySQL tables.

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