How to Efficiently Retrieve Distinct Values from Multiple Columns in MySQL?

Identifying Distinct Values in Multiple Columns Using DISTINCT in MySQL

In MySQL, you may encounter a scenario where you need to retrieve only the unique values from multiple columns in a table. However, using the DISTINCT keyword alone may not always provide the desired results. This article aims to clarify this issue and provide a solution for selecting distinct values for multiple columns separately.

As you mentioned, using SELECT DISTINCT a,b,c,d FROM my_table would not work because it would only return rows where all four columns have distinct values. This is not what you want, as you need the distinct values for each column separately.

To achieve this, you can leverage the GROUP BY clause in combination with DISTINCT. Here's how it works:

SELECT DISTINCT a, b
FROM my_table
GROUP BY a, b;

In this query, the DISTINCT keyword is applied to a and b within the GROUP BY clause. This means that for each unique combination of a and b values in the table, the query will only return a single row. The result will be a list of all distinct pairs of a and b values.

You can add additional columns to the GROUP BY clause if needed. For example, to get the distinct values of all four columns a, b, c, and d, you would use:

SELECT DISTINCT a, b, c, d
FROM my_table
GROUP BY a, b, c, d;

This will give you a list of all the unique combinations of those four columns in the table.

It's important to note that this approach will return distinct combinations of values, not necessarily the values from one specific row. If you want to ensure that only one row is returned for each group, you can add an additional LIMIT 1 clause after the GROUP BY clause.

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