In MySQL, the DISTINCT keyword can be applied to all columns. When you use DISTINCT, it will eliminate duplicate rows from the query results and return only unique rows.
The underlying principle is that MySQL performs the following steps to handle DISTINCT:
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MySQL first executes the FROM clause to determine the table to retrieve data from.
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Then, MySQL applies the WHERE clause to filter the eligible rows.
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Next, MySQL executes the SELECT clause to select the columns to return. When you use DISTINCT, MySQL will operate in this step.
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MySQL uses the selected column to compare the values in each row to find duplicate rows.
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Once duplicate rows are found, MySQL returns only one of them and ignores other duplicate rows.
It should be noted that the DISTINCT keyword acts on the combination of all selected columns, not on each column independently. This means that if you select multiple columns, MySQL will consider combinations of those columns, not just unique values for each column.
In addition, it should be noted that using DISTINCT may have a certain impact on the performance of the query, especially when dealing with large amounts of data. This is because MySQL needs to sort and deduplicate the returned results. If query performance becomes an issue, you may consider using other optimization techniques such as indexes or subqueries to improve query performance.